change build_dir build test to test build_dir specified as a Regexp

build_tests/build_dir/build.rb

@@ -1,5 +1,5 @@
 Rscons::Environment.new do |env|
   env.append('CPPPATH' => Dir['src/**/*/'])
-  env.build_dir('src', 'build')
+  env.build_dir(%r{^src/([^/]+)/}, 'build_\\1/')
   env.Program('build_dir', Dir['src/**/*.c'])
 end