* Qt5::Test is not required in the global QT_LIBRARIES definition
* Qt5::DBus had already been optional, drop bogus pkgconfig search
This partially reverts commit 4321ecf7d2.
* Find X11 only once, in root CMakeLists.txt
Since we have HAVE_X11, use HAVE_X11 in cmake.
One usage of the macro remains in the bundled libmygpo-qt5. This should
be updated once the pending upstream fix was merged. qtsingleapplication
is already fixed upstream.