add Environment#build_root to be used if no build directories match

lib/rscons/environment.rb

@@ -9,6 +9,9 @@ module Rscons
     # Hash of +{"builder_name" => builder_object}+ pairs.
     attr_reader :builders
 
+    # String or +nil+
+    attr_accessor :build_root
+
     # Create an Environment object.
     # @param variables [Hash]
     #   The variables hash can contain construction variables, which are
@@ -88,14 +91,19 @@ module Rscons
     # This method takes into account the Environment's build directories.
     def get_build_fname(source_fname, suffix)
       build_fname = source_fname.set_suffix(suffix).gsub('\\', '/')
-      @build_dirs.each do |src_dir, obj_dir|
+      found_match = @build_dirs.find do |src_dir, obj_dir|
         if src_dir.is_a?(Regexp)
           build_fname.sub!(src_dir, obj_dir)
         else
           build_fname.sub!(%r{^#{src_dir}/}, "#{obj_dir}/")
         end
-        build_fname.gsub!('\\', '/')
       end
+      if @build_root and not found_match
+        unless source_fname.start_with?('/') or source_fname =~ %r{^\w:[\\/]}
+          build_fname = "#{@build_root}/#{build_fname}"
+        end
+      end
+      build_fname.gsub!('\\', '/')
       build_fname
     end
 

spec/build_tests_spec.rb

@@ -158,6 +158,28 @@ describe Rscons do
     File.exists?('build_two/two.o').should be_true
   end
 
+  it 'uses build directories before build root' do
+    test_dir('build_dir')
+    Rscons::Environment.new do |env|
+      env.append('CPPPATH' => Dir['src/**/*/'])
+      env.build_dir("src", "build")
+      env.build_root = "build_root"
+      env.Program('build_dir', Dir['src/**/*.c'])
+    end
+    lines.should == ["CC build/one/one.o", "CC build/two/two.o", "LD build_dir"]
+  end
+
+  it 'uses build_root if no build directories match' do
+    test_dir('build_dir')
+    Rscons::Environment.new do |env|
+      env.append('CPPPATH' => Dir['src/**/*/'])
+      env.build_dir("src2", "build")
+      env.build_root = "build_root"
+      env.Program('build_dir', Dir['src/**/*.c'])
+    end
+    lines.should == ["CC build_root/src/one/one.o", "CC build_root/src/two/two.o", "LD build_dir"]
+  end
+
   it 'cleans built files' do
     test_dir('build_dir')
     Rscons::Environment.new do |env|