print the error message before abort()
compile /a/ as ::Regexp.new("a")
fix access denied error at incremental build
Clean up mrbconf.h

- undef for windows is not used by default so comment it
- casting to the same type is not needed
- parenthesis are not needed for macro attributes separated by commas
- change order of MRB_NAN_BOXING and MRB_INT64 to simplify logic
Merge pull request #864 from nkshigeru/build_for_vc

fix access denied error at incremental build
Merge pull request #865 from brainopia/mrbconf

Clean up mrbconf.h
remove unused HAVE_UNISTD_H altogether; ref #865
better comment description for mrbconf.h configuration