fully parallelize the Command builder

build_tests/simple/command_builder.rb

@@ -0,0 +1,7 @@
+Rscons::Environment.new do |env|
+  command = %W[gcc -o ${_TARGET} ${_SOURCES}]
+  env.Command("simple.exe",
+              "simple.c",
+              "CMD" => command,
+              "CMD_DESC" => "BuildIt")
+end