Workaround for launcher compilation problems due to bug in Qt MOC compiler.

For details please see:

https://forum.openmw.org/viewtopic.php?f=7&t=1451
https://bugreports.qt-project.org/browse/QTBUG-22829

Signed-off-by: Lukasz Gromanowski <lgromanowski@gmail.com>
Merge remote-tracking branch 'lgro/qt_and_boost_join_workaround'