qt5/base: prepare for own mkspec file for taregt specific host builds

qt5/base: prepare for own mkspec file for taregt specific host builds

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.