identify Qt using libc++ on mac and don't let people use g++ with them

on Mac g++ is quite outdated and has some issue with the current packages https://bugreports.qt-project.org/browse/QTBUG-38193 .
That might be solved at qt level soon (removing one mkspec), but in the meantime one can compile and link qt with libc++, and then any function (like toStdString) that returns std types might crash horribly.

I think that we should identify the linked C++ lib (either from qmake flags or otool -L and warn accordingly) when one tries to mix compilers especially if using c++11.