Merge pull request #1215 from monaka/pr-avoid-define-MRB_INT16-and-MRB_INT64-at-the-same-time

Avoid to define MRB_INT16 and MRB_INT64 at the same time
Merge pull request #1216 from monaka/pr-code-cleanup-20130426

Code clean-up
quote error message in mrbconf.h to stop warning