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

[tobil4sk]

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 cppflags and mmflags, it might be worth considering enabling all cppflags 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