Replace 0 with NULL because struct pointer is set NULL in.
Replace int with mrb_bool because a return value is boolean.
Merge pull request #1381 from suzukaze/refactor2-pool.c

Replace int with mrb_bool because a return value is boolean.
Merge pull request #1380 from suzukaze/refactor-pool.c

Replace 0 with NULL because struct pointer is set NULL in.
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