Allow programmers to specify build path manually
Repalace int with mrb_bool because a return value is boolean.
Merge pull request #1385 from suzukaze/refactor-return

Repalace int with mrb_bool because a return value is boolean.
Fix warning by making call explicit
Merge pull request #1384 from xxuejie/customize-build-dir

Allow programmers to specify build path manually
Merge pull request #1388 from carsonmcdonald/toplevelwarnfix

Fix warning by making call explicit