diff --git a/lib/rscons.rb b/lib/rscons.rb
index bf8a58a..bcf1fe9 100644
--- a/lib/rscons.rb
+++ b/lib/rscons.rb
@@ -12,20 +12,6 @@ require_relative "rscons/util"
 require_relative "rscons/varset"
 require_relative "rscons/version"
 
-# default builders
-require_relative "rscons/builders/cfile"
-require_relative "rscons/builders/command"
-require_relative "rscons/builders/directory"
-require_relative "rscons/builders/disassemble"
-require_relative "rscons/builders/install"
-require_relative "rscons/builders/library"
-require_relative "rscons/builders/object"
-require_relative "rscons/builders/preprocess"
-require_relative "rscons/builders/program"
-require_relative "rscons/builders/shared_library"
-require_relative "rscons/builders/shared_object"
-require_relative "rscons/builders/simple_builder"
-
 # Namespace module for rscons classes
 module Rscons
 
@@ -177,5 +163,19 @@ module Rscons
 
 end
 
+# default builders
+require_relative "rscons/builders/cfile"
+require_relative "rscons/builders/command"
+require_relative "rscons/builders/directory"
+require_relative "rscons/builders/disassemble"
+require_relative "rscons/builders/install"
+require_relative "rscons/builders/library"
+require_relative "rscons/builders/object"
+require_relative "rscons/builders/preprocess"
+require_relative "rscons/builders/program"
+require_relative "rscons/builders/shared_library"
+require_relative "rscons/builders/shared_object"
+require_relative "rscons/builders/simple_builder"
+
 # Unbuffer $stdout
 $stdout.sync = true
diff --git a/lib/rscons/application.rb b/lib/rscons/application.rb
index e61c0e1..f1d9ccd 100644
--- a/lib/rscons/application.rb
+++ b/lib/rscons/application.rb
@@ -15,8 +15,14 @@ module Rscons
     #   Access any variables set on the rscons command-line.
     attr_reader :vars
 
+    # @return [VarSet]
+    #   The default construction variables provided by builders.
+    attr_reader :default_varset
+
+    # Create Application instance.
     def initialize
       @n_threads = determine_n_threads
+      @default_varset = VarSet.new
       @vars = VarSet.new
     end
 
diff --git a/lib/rscons/builder.rb b/lib/rscons/builder.rb
index bae8905..06da747 100644
--- a/lib/rscons/builder.rb
+++ b/lib/rscons/builder.rb
@@ -15,15 +15,6 @@ module Rscons
       self.class.name.split(":").last
     end
 
-    # Return a set of default construction variables for the builder.
-    #
-    # @param env [Environment] The Environment.
-    #
-    # @return [Hash] Default construction variables.
-    def default_variables(env)
-      {}
-    end
-
     # Return a set of build features that this builder provides.
     #
     # @return [Array<String>]
diff --git a/lib/rscons/builders/cfile.rb b/lib/rscons/builders/cfile.rb
index e8c061f..470b780 100644
--- a/lib/rscons/builders/cfile.rb
+++ b/lib/rscons/builders/cfile.rb
@@ -8,23 +8,16 @@ module Rscons
     #   env.CFile("lex.yy.cc", "parser.ll")
     class CFile < Builder
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          "YACC" => "bison",
-          "YACC_FLAGS" => ["-d"],
-          "YACC_CMD" => ["${YACC}", "${YACC_FLAGS}", "-o", "${_TARGET}", "${_SOURCES}"],
-          "YACCSUFFIX" => [".y", ".yy"],
-          "LEX" => "flex",
-          "LEX_FLAGS" => [],
-          "LEX_CMD" => ["${LEX}", "${LEX_FLAGS}", "-o", "${_TARGET}", "${_SOURCES}"],
-          "LEXSUFFIX" => [".l", ".ll"],
-        }
-      end
+      Rscons.application.default_varset.append(
+        "YACC" => "bison",
+        "YACC_FLAGS" => ["-d"],
+        "YACC_CMD" => ["${YACC}", "${YACC_FLAGS}", "-o", "${_TARGET}", "${_SOURCES}"],
+        "YACCSUFFIX" => [".y", ".yy"],
+        "LEX" => "flex",
+        "LEX_FLAGS" => [],
+        "LEX_CMD" => ["${LEX}", "${LEX_FLAGS}", "-o", "${_TARGET}", "${_SOURCES}"],
+        "LEXSUFFIX" => [".l", ".ll"],
+      )
 
       # Run the builder to produce a build target.
       #
diff --git a/lib/rscons/builders/disassemble.rb b/lib/rscons/builders/disassemble.rb
index ddbecae..a1aa40b 100644
--- a/lib/rscons/builders/disassemble.rb
+++ b/lib/rscons/builders/disassemble.rb
@@ -3,18 +3,11 @@ module Rscons
     # The Disassemble builder produces a disassembly listing of a source file.
     class Disassemble < Builder
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          "OBJDUMP" => "objdump",
-          "DISASM_CMD" => ["${OBJDUMP}", "${DISASM_FLAGS}", "${_SOURCES}"],
-          "DISASM_FLAGS" => ["--disassemble", "--source"],
-        }
-      end
+      Rscons.application.default_varset.append(
+        "OBJDUMP" => "objdump",
+        "DISASM_CMD" => ["${OBJDUMP}", "${DISASM_FLAGS}", "${_SOURCES}"],
+        "DISASM_FLAGS" => ["--disassemble", "--source"],
+      )
 
       # Run the builder to produce a build target.
       #
diff --git a/lib/rscons/builders/library.rb b/lib/rscons/builders/library.rb
index a8b2ad9..99880c5 100644
--- a/lib/rscons/builders/library.rb
+++ b/lib/rscons/builders/library.rb
@@ -3,19 +3,12 @@ module Rscons
     # A default Rscons builder that produces a static library archive.
     class Library < Builder
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          'AR' => 'ar',
-          'LIBSUFFIX' => '.a',
-          'ARFLAGS' => ['rcs'],
-          'ARCMD' => ['${AR}', '${ARFLAGS}', '${_TARGET}', '${_SOURCES}']
-        }
-      end
+      Rscons.application.default_varset.append(
+        'AR' => 'ar',
+        'LIBSUFFIX' => '.a',
+        'ARFLAGS' => ['rcs'],
+        'ARCMD' => ['${AR}', '${ARFLAGS}', '${_TARGET}', '${_SOURCES}']
+      )
 
       # Set up a build operation using this builder.
       #
diff --git a/lib/rscons/builders/object.rb b/lib/rscons/builders/object.rb
index 24c6e76..663bad1 100644
--- a/lib/rscons/builders/object.rb
+++ b/lib/rscons/builders/object.rb
@@ -12,53 +12,46 @@ module Rscons
         "DC" => "DSUFFIX",
       }
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          'OBJSUFFIX' => ['.o'],
-          'DEPFILESUFFIX' => '.mf',
+      Rscons.application.default_varset.append(
+        'OBJSUFFIX' => ['.o'],
+        'DEPFILESUFFIX' => '.mf',
 
-          'CPPDEFPREFIX' => '-D',
-          'INCPREFIX' => '-I',
+        'CPPDEFPREFIX' => '-D',
+        'INCPREFIX' => '-I',
 
-          'AS' => '${CC}',
-          'ASFLAGS' => [],
-          'ASSUFFIX' => ['.S'],
-          'ASPPPATH' => '${CPPPATH}',
-          'ASPPFLAGS' => '${CPPFLAGS}',
-          'ASDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
-          'ASCMD' => ['${AS}', '-c', '-o', '${_TARGET}', '${ASDEPGEN}', '${INCPREFIX}${ASPPPATH}', '${ASPPFLAGS}', '${ASFLAGS}', '${_SOURCES}'],
+        'AS' => '${CC}',
+        'ASFLAGS' => [],
+        'ASSUFFIX' => ['.S'],
+        'ASPPPATH' => '${CPPPATH}',
+        'ASPPFLAGS' => '${CPPFLAGS}',
+        'ASDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
+        'ASCMD' => ['${AS}', '-c', '-o', '${_TARGET}', '${ASDEPGEN}', '${INCPREFIX}${ASPPPATH}', '${ASPPFLAGS}', '${ASFLAGS}', '${_SOURCES}'],
 
-          'CPPFLAGS' => ['${CPPDEFPREFIX}${CPPDEFINES}'],
-          'CPPDEFINES' => [],
-          'CPPPATH' => [],
+        'CPPFLAGS' => ['${CPPDEFPREFIX}${CPPDEFINES}'],
+        'CPPDEFINES' => [],
+        'CPPPATH' => [],
 
-          'CCFLAGS' => [],
+        'CCFLAGS' => [],
 
-          'CC' => 'gcc',
-          'CFLAGS' => [],
-          'CSUFFIX' => ['.c'],
-          'CCDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
-          'CCCMD' => ['${CC}', '-c', '-o', '${_TARGET}', '${CCDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${CFLAGS}', '${CCFLAGS}', '${_SOURCES}'],
+        'CC' => 'gcc',
+        'CFLAGS' => [],
+        'CSUFFIX' => ['.c'],
+        'CCDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
+        'CCCMD' => ['${CC}', '-c', '-o', '${_TARGET}', '${CCDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${CFLAGS}', '${CCFLAGS}', '${_SOURCES}'],
 
-          'CXX' => 'g++',
-          'CXXFLAGS' => [],
-          'CXXSUFFIX' => ['.cc', '.cpp', '.cxx', '.C'],
-          'CXXDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
-          'CXXCMD' =>['${CXX}', '-c', '-o', '${_TARGET}', '${CXXDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${CXXFLAGS}', '${CCFLAGS}', '${_SOURCES}'],
+        'CXX' => 'g++',
+        'CXXFLAGS' => [],
+        'CXXSUFFIX' => ['.cc', '.cpp', '.cxx', '.C'],
+        'CXXDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
+        'CXXCMD' =>['${CXX}', '-c', '-o', '${_TARGET}', '${CXXDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${CXXFLAGS}', '${CCFLAGS}', '${_SOURCES}'],
 
-          'DC' => 'gdc',
-          'DFLAGS' => [],
-          'DSUFFIX' => ['.d'],
-          'DDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
-          'D_IMPORT_PATH' => [],
-          'DCCMD' => ['${DC}', '-c', '-o', '${_TARGET}', '${DDEPGEN}', '${INCPREFIX}${D_IMPORT_PATH}', '${DFLAGS}', '${_SOURCES}'],
-        }
-      end
+        'DC' => 'gdc',
+        'DFLAGS' => [],
+        'DSUFFIX' => ['.d'],
+        'DDEPGEN' => ['-MMD', '-MF', '${_DEPFILE}'],
+        'D_IMPORT_PATH' => [],
+        'DCCMD' => ['${DC}', '-c', '-o', '${_TARGET}', '${DDEPGEN}', '${INCPREFIX}${D_IMPORT_PATH}', '${DFLAGS}', '${_SOURCES}'],
+      )
 
       # Return whether this builder object is capable of producing a given target
       # file name from a given source file name.
diff --git a/lib/rscons/builders/preprocess.rb b/lib/rscons/builders/preprocess.rb
index baf9e27..a51dc33 100644
--- a/lib/rscons/builders/preprocess.rb
+++ b/lib/rscons/builders/preprocess.rb
@@ -5,18 +5,11 @@ module Rscons
     # The Preprocess builder invokes the C preprocessor
     class Preprocess < Builder
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          "CPP_CMD" => %w[
-            ${_PREPROCESS_CC} -E ${_PREPROCESS_DEPGEN}
-            -o ${_TARGET} ${INCPREFIX}${CPPPATH} ${CPPFLAGS} ${_SOURCES}],
-        }
-      end
+      Rscons.application.default_varset.append(
+        "CPP_CMD" => %w[
+          ${_PREPROCESS_CC} -E ${_PREPROCESS_DEPGEN}
+          -o ${_TARGET} ${INCPREFIX}${CPPPATH} ${CPPFLAGS} ${_SOURCES}],
+      )
 
       # Run the builder to produce a build target.
       #
diff --git a/lib/rscons/builders/program.rb b/lib/rscons/builders/program.rb
index cefdf92..ea541d9 100644
--- a/lib/rscons/builders/program.rb
+++ b/lib/rscons/builders/program.rb
@@ -4,25 +4,18 @@ module Rscons
     # executable program.
     class Program < Builder
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          'OBJSUFFIX' => '.o',
-          'PROGSUFFIX' => (Object.const_get("RUBY_PLATFORM") =~ /mingw|cygwin/ ? ".exe" : ""),
-          'LD' => nil,
-          'LIBSUFFIX' => '.a',
-          'LDFLAGS' => [],
-          'LIBPATH' => [],
-          'LIBDIRPREFIX' => '-L',
-          'LIBLINKPREFIX' => '-l',
-          'LIBS' => [],
-          'LDCMD' => ['${LD}', '-o', '${_TARGET}', '${LDFLAGS}', '${_SOURCES}', '${LIBDIRPREFIX}${LIBPATH}', '${LIBLINKPREFIX}${LIBS}']
-        }
-      end
+      Rscons.application.default_varset.append(
+        'OBJSUFFIX' => '.o',
+        'PROGSUFFIX' => (Object.const_get("RUBY_PLATFORM") =~ /mingw|cygwin/ ? ".exe" : ""),
+        'LD' => nil,
+        'LIBSUFFIX' => '.a',
+        'LDFLAGS' => [],
+        'LIBPATH' => [],
+        'LIBDIRPREFIX' => '-L',
+        'LIBLINKPREFIX' => '-l',
+        'LIBS' => [],
+        'LDCMD' => ['${LD}', '-o', '${_TARGET}', '${LDFLAGS}', '${_SOURCES}', '${LIBDIRPREFIX}${LIBPATH}', '${LIBLINKPREFIX}${LIBS}']
+      )
 
       # Create a BuildTarget object for this build target.
       #
diff --git a/lib/rscons/builders/shared_library.rb b/lib/rscons/builders/shared_library.rb
index 7c05620..a7912a3 100644
--- a/lib/rscons/builders/shared_library.rb
+++ b/lib/rscons/builders/shared_library.rb
@@ -4,22 +4,15 @@ module Rscons
     # shared library.
     class SharedLibrary < Builder
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        {
-          'SHLIBPREFIX' => (RUBY_PLATFORM =~ /mingw/ ? '' : 'lib'),
-          'SHLIBSUFFIX' => (RUBY_PLATFORM =~ /mingw/ ? '.dll' : '.so'),
-          'SHLDFLAGS' => ['${LDFLAGS}', '-shared'],
-          'SHLD' => nil,
-          'SHLIBDIRPREFIX' => '-L',
-          'SHLIBLINKPREFIX' => '-l',
-          'SHLDCMD' => ['${SHLD}', '-o', '${_TARGET}', '${SHLDFLAGS}', '${_SOURCES}', '${SHLIBDIRPREFIX}${LIBPATH}', '${SHLIBLINKPREFIX}${LIBS}']
-        }
-      end
+      Rscons.application.default_varset.append(
+        'SHLIBPREFIX' => (RUBY_PLATFORM =~ /mingw/ ? '' : 'lib'),
+        'SHLIBSUFFIX' => (RUBY_PLATFORM =~ /mingw/ ? '.dll' : '.so'),
+        'SHLDFLAGS' => ['${LDFLAGS}', '-shared'],
+        'SHLD' => nil,
+        'SHLIBDIRPREFIX' => '-L',
+        'SHLIBLINKPREFIX' => '-l',
+        'SHLDCMD' => ['${SHLD}', '-o', '${_TARGET}', '${SHLDFLAGS}', '${_SOURCES}', '${SHLIBDIRPREFIX}${LIBPATH}', '${SHLIBLINKPREFIX}${LIBS}']
+      )
 
       # Return a set of build features that this builder provides.
       #
diff --git a/lib/rscons/builders/shared_object.rb b/lib/rscons/builders/shared_object.rb
index 97584c5..2c1cdff 100644
--- a/lib/rscons/builders/shared_object.rb
+++ b/lib/rscons/builders/shared_object.rb
@@ -13,29 +13,22 @@ module Rscons
         "SHDC" => "DSUFFIX",
       }
 
-      # Return default construction variables for the builder.
-      #
-      # @param env [Environment] The Environment using the builder.
-      #
-      # @return [Hash] Default construction variables for the builder.
-      def default_variables(env)
-        pic_flags = (RUBY_PLATFORM =~ /mingw/ ? [] : ['-fPIC'])
-        {
-          'SHCCFLAGS' => ['${CCFLAGS}'] + pic_flags,
+      pic_flags = (RUBY_PLATFORM =~ /mingw/ ? [] : ['-fPIC'])
+      Rscons.application.default_varset.append(
+        'SHCCFLAGS' => ['${CCFLAGS}'] + pic_flags,
 
-          'SHCC' => '${CC}',
-          'SHCFLAGS' => ['${CFLAGS}'],
-          'SHCCCMD' => ['${SHCC}', '-c', '-o', '${_TARGET}', '${CCDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${SHCFLAGS}', '${SHCCFLAGS}', '${_SOURCES}'],
+        'SHCC' => '${CC}',
+        'SHCFLAGS' => ['${CFLAGS}'],
+        'SHCCCMD' => ['${SHCC}', '-c', '-o', '${_TARGET}', '${CCDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${SHCFLAGS}', '${SHCCFLAGS}', '${_SOURCES}'],
 
-          'SHCXX' => '${CXX}',
-          'SHCXXFLAGS' => ['${CXXFLAGS}'],
-          'SHCXXCMD' => ['${SHCXX}', '-c', '-o', '${_TARGET}', '${CXXDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${SHCXXFLAGS}', '${SHCCFLAGS}', '${_SOURCES}'],
+        'SHCXX' => '${CXX}',
+        'SHCXXFLAGS' => ['${CXXFLAGS}'],
+        'SHCXXCMD' => ['${SHCXX}', '-c', '-o', '${_TARGET}', '${CXXDEPGEN}', '${INCPREFIX}${CPPPATH}', '${CPPFLAGS}', '${SHCXXFLAGS}', '${SHCCFLAGS}', '${_SOURCES}'],
 
-          'SHDC' => 'gdc',
-          'SHDFLAGS' => ['${DFLAGS}'] + pic_flags,
-          'SHDCCMD' => ['${SHDC}', '-c', '-o', '${_TARGET}', '${INCPREFIX}${D_IMPORT_PATH}', '${SHDFLAGS}', '${_SOURCES}'],
-        }
-      end
+        'SHDC' => 'gdc',
+        'SHDFLAGS' => ['${DFLAGS}'] + pic_flags,
+        'SHDCCMD' => ['${SHDC}', '-c', '-o', '${_TARGET}', '${INCPREFIX}${D_IMPORT_PATH}', '${SHDFLAGS}', '${_SOURCES}'],
+      )
 
       # Return a set of build features that this builder provides.
       #
diff --git a/lib/rscons/environment.rb b/lib/rscons/environment.rb
index 7a56689..14e681d 100644
--- a/lib/rscons/environment.rb
+++ b/lib/rscons/environment.rb
@@ -69,7 +69,7 @@ module Rscons
       @threaded_commands = Set.new
       @registered_build_dependencies = {}
       @side_effects = {}
-      @varset = VarSet.new
+      @varset = VarSet.new(Rscons.application.default_varset)
       @job_set = JobSet.new(@registered_build_dependencies, @side_effects)
       @user_deps = {}
       @builders = {}
@@ -169,12 +169,6 @@ module Rscons
         builder = Rscons::Builders::SimpleBuilder.new(builder, &action)
       end
       @builders[builder.name] = builder
-      var_defs = builder.default_variables(self)
-      if var_defs
-        var_defs.each_pair do |var, val|
-          @varset[var] ||= val
-        end
-      end
     end
 
     # Add a build hook to the Environment.