c++ standard flags are ignored by mac toolchain for objective c++ files

When using the mac toolchain to compile .mm files, `HXCPP_CPP11` is ignored by hxcpp, so no `-std=c++11` flag is added to the build. Since there is already a lot of duplication between `cppflag`s and `mmflag`s, it might be worth considering enabling all `cppflag`s in objective c++ compilation, perhaps with an attribute to turn that off per flag.

See:
https://github.com/HaxeFoundation/hxcpp/blob/be7ef02b785cf9cac8377b0ddbeef5026aa65221/toolchain/mac-toolchain.xml#L29