use bundle to manage gems in configure

[mrbrdo]

use "bundle install" to install gem dependencies during configure, and check return status of this command to know if gems are installed correctly (if they are not already installed they will be installed). To test error return status, add a line like gem 'blablablablabla' to the Gemfile, then bundle install should fail.

this way you don't need to check yourself if the gems are installed. but instead of that just check if the bundle command is in path and then just run bundle install in root dir.

I added the Gemfile in this commit: b354549

If you have hub (https://github.com/defunkt/hub) use

hub am https://github.com/mrbrdo/mruby_cc/commit/b3545497fa29b41397175b0f4f035f4391516c6a

[mrbrdo]

@slivnik