Fix building qmake when doing a Qt static build

Currently building qmake fails in a static build due to duplicate symbols, which come from individual cpp files and the QtCore library. The dynamic linker is smart enough to pick only one symbol when doing a shared library Qt build, but not in the static case.

We should modify the cpp files after we merge the wip/cmake branch into qt6 branch, to fix qmake to be built in a static build. We shouldn't do it before the merge, because the cpp file changes would probably break the qmake Qt build.

Alternatively we could choose not to link qmake against QtCore, but we would like to avoid that.