Cmake build: Use find_package(Qt5X11Extras ...) independent of OS

We need to know, if Qt5 is built over X11 in order to test with
xvkbd, which manipulates XEvents.
Qt5X11Extras is independent of underlying OS.
Merge remote-tracking branch 'github/build-bot-master' into bleeding-edge-master
2014.08.23-10.00