Always include build_config.rb to compile dependency.

build_config.rb modifies `cc.defines` (eg. `DISABLE_GEMS`) so files depending on such flags should be rebuilt.

 1. rake without any gems
 2. edit build_config.rb to add gem
 3. rake (without rake clean)

rake output `Build summary` with `Included Gems` but actually `DISABLE_GEMS` flag lives.

Ref. (Failing scenario script): https://gist.github.com/cho45/9181191