Fix #448 and minor build system issues

[yyamano]

• Remove mruby/build/* if mruby source is newer. Fix Symbol not found: _mrb_sym2str #448
• Don't remove mruby/build/* in build.sh. It is done by Makefile if necessary.
• Invoke rake with our own build_config.rb.

Pull-Request Check List

• Add patches into src/.
• Add test into test/. Please see about test docs.
• Add docs into docs/ if you change the features such as build system, Ruby methods, class and nginx directives.

[matsumotory]

lgtm!