This is trying to address the following issue: We pass variables to
qmake which get appended to qmake specific vars in mkspecs/*/qmake.conf.
However projects might invoke qmake once again, however without passing
respective variables along.
We might need to consider hard-coding the flags inside qmake.conf.
That way projects not using qmake (but cmake) can still import all qt5
specific variables and helper functions, without its routines being
overriden by the qmake specific ones.
