improve build system in order to compile spec files in parallel as much as possible

So far, RubyMotion has two phase in compile when run spec (1. compile app files, 2. compile spec files).
it make idle cpu cores when finished to compile app files (before starting to compile spec files).

lib/motion/project/builder.rb

@@ -165,22 +165,18 @@ module Motion; module Project;
 
       parallel = ParallelBuilder.new(objs_build_dir, build_file)
       parallel.files = config.ordered_build_files
+      parallel.files += config.spec_files if config.spec_mode
       parallel.run
-      objs = parallel.objects
 
-      FileUtils.touch(objs_build_dir) if any_obj_file_built
-
-      app_objs = objs
+      objs = app_objs = parallel.objects
       spec_objs = []
       if config.spec_mode
-        # Build spec files too, but sequentially.
-        parallel = ParallelBuilder.new(objs_build_dir, build_file)
-        parallel.files = config.spec_files
-        parallel.run
-        spec_objs = parallel.objects
-        objs += spec_objs
+        app_objs = objs[0...config.ordered_build_files.size]
+        spec_objs = objs[-(config.spec_files.size)..-1]
       end
 
+      FileUtils.touch(objs_build_dir) if any_obj_file_built
+
       # Generate init file.
       init_txt = <<EOS
 extern "C" {