Qmake builds for MSVC.

These last two lines should not be needed at all - the relevant flags _should_ be pulled in automatically from the mkspecs files used when qmake is run, although the choice of -O2 or -O3 for a CONFIG+=release build does depend a little on the platform that a GCC compiler is running on (and if one is optimising for size not speed you'd be using -Os instead but there is no straightforward way to set that via a CONFIG option).