qmake doesn't treat /openmp- properly

qmake only checks for the 'open' prefix and then sets OpenMP to true @ http://qt.gitorious.org/qt/qt/blobs/4.8/qmake/generators/win32/msvc_objectmodel.cpp#line1082

But there also is the /openmp- variant to explicitly disable the option.

There might be more such issues in the code since it often only checks the prefix.