libstdc++ removed from XCode 10, compile of mjdgges fails on MacOS Mojave

I had the bad idea to upgrade to Mojave (or to upgrade XCode, not clear ...): it seems libstdc++ is no longer there. after changing on the command line the compilation call in the build/matlab/mjdgges directory, by using

-lc++

instead of

-lstdc++

compilation went OK. Perhaps we should add detection for Darwin and use 'libc++' if detected ?