Fix `install` target on OSX
Merge pull request #66 from riywo/make_install_osx

Fix `install` target on OSX
Add the method of setting variable for syntax sugar.

v.set "hoge", v.hoge.to_i * 2

is equal below method:

v.hoge =  v.hoge.to_i * 2
git checkout build_config.rb after make build_mruby
Merge pull request #67 from riywo/make_build_mruby_git_checkout

git checkout build_config.rb after make build_mruby