Compatibility with versions prior to 2.8.12 is being deprecated,
resulting in build warning messages. The minimum for the official
supported distros is 3.7.2 (Stretch), so the version could be moved
forward when newer features are required.
Reference: https://cmake.org/cmake/help/v3.19/release/3.19.html#deprecated-and-removed-features
* 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.
