Although it's already quite ugly that we overwrite QMAKE_CC like this,
hardcoding 'g++' can break the build where prefixed versions of
the toolchain are expected and where QMAKE_CXX is already properly
defined. So we just use QMAKE_CXX variable here.
This fixes build failures on some distributions, e.g.
https://galileo.mailstation.de/jenkins/job/lipidity/36/console
