change "vars" parameters to be required

lib/rscons/builders/library.rb

@@ -12,7 +12,7 @@ module Rscons
       }
     end
 
-    def run(target, sources, cache, env, vars = {})
+    def run(target, sources, cache, env, vars)
       # build sources to linkable objects
       objects = env.build_sources(sources, [env['OBJSUFFIX'], env['LIBSUFFIX']].flatten, cache, vars)
       if objects

lib/rscons/builders/object.rb

@@ -50,7 +50,7 @@ module Rscons
       end
     end
 
-    def run(target, sources, cache, env, vars = {})
+    def run(target, sources, cache, env, vars)
       vars = vars.merge({
         '_TARGET' => target,
         '_SOURCES' => sources,

lib/rscons/builders/program.rb

@@ -14,7 +14,7 @@ module Rscons
       }
     end
 
-    def run(target, sources, cache, env, vars = {})
+    def run(target, sources, cache, env, vars)
       # build sources to linkable objects
       objects = env.build_sources(sources, [env['OBJSUFFIX'], env['LIBSUFFIX']].flatten, cache, vars)
       return false unless objects

lib/rscons/environment.rb

@@ -228,7 +228,7 @@ module Rscons
     # @param cache [Cache] The Cache.
     # @param vars [Hash] Extra variables to pass to the builder.
     # Return a list of the converted file names.
-    def build_sources(sources, suffixes, cache, vars = {})
+    def build_sources(sources, suffixes, cache, vars)
       sources.map do |source|
         if source.has_suffix?(suffixes)
           source