[dev-qt/qtwebkit:5] Respect C(XX)FLAGS

Flags were loaded from /usr/lib64/qt5/mkspecs/qmodule.pri before. This overwrites them here before QtWebkit does its own additions to C(XX)FLAGS.

I'm not really convinced that this is the best solution but I was not able to figure out a better alternative. (E.g. it would be nice to have a kind of "file overlay" on /usr/lib64/qt5/mkspecs/qmodule.pri where we have specified the correct C(XX)FLAGS)

This has to be done for every package that is a Qt module but not part of the qtbase tarball.

Fixes https://bugs.gentoo.org/show_bug.cgi?id=490254 for qtwebkit:5