move dependency check from runtime to mrbgem.rake
remove new_label(s) where label position can be obtained from genop()
fix compile error check of Mrbc command
Merge pull request #1855 from take-cheeze/fix_mrbc_compile_error

Fix compile error check of Mrbc command.
Merge branch 'master' of github.com:mruby/mruby
Don't redefine timeval on MingW64 environments
Merge pull request #1857 from mitchellh/f-fix-mingw-time

Don't redefine timeval in MinGW environments