Changed flags name in libmruby.flags.mak
Added "Unknown toolchain/gem" error message
Merge pull request #763 from matsumoto-r/change_cflags_name

Changed flags name in libmruby.flags.mak
Merge pull request #767 from masuidrive/build_error_message

Added "Unknown toolchain/gem" error message
zero initialize hex buffers; close 768
Merge branch 'master' of github.com:mruby/mruby