Rebuild CRuby based building script without Makefile

Tested CRuby 1.8.6 and 1.9.3

You can see building configuration in build_config.rb

tools/mruby/mruby.rake

@@ -0,0 +1,10 @@
+dir = File.dirname(__FILE__).sub(%r|^\./|, '')
+
+MRuby.each_target do
+  exec = exefile("#{build_dir}/bin/mruby")
+  objs = Dir.glob("#{dir}/*.{c}").map { |f| f.pathmap("#{build_dir}/%X.o") }
+
+  file exec => objs + ["#{build_dir}/lib/libmruby.a"] do |t|
+    link t.name, t.prerequisites, [], gems.map { |g| g.mruby_libs }
+  end
+end