-build-with-qt6 did not work for me, because the flag is
ignored when selecting the qmake executable. It would find
the system-wide qmake executable, which is Qt5 and then
decide to build with Qt5.
When the flag is set, try to search for a Qt6 version of
qmake first. On Ubuntu based distros this seems to be
qmake6
Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>