* X11 is optional dependency
* XCB is optional dependency
* Qt::X11Extras is only found if both X11 and XCB are found
* switch to HAVE_X11 instead of X11_FOUND in CMakeLists
* remove/fix custom added definitions
* use #if HAVE_X11 instead of #ifdef HAVE_X11 (that is always true)
REVIEW: 115698