The build files had quite a few things wrong:
* Not using the method: kwarg, which can cause the wrong Qt to be
used for building
* There was no way to enable the build for them
* Qt was being detected multiple times, differently
* Unnecessary check for libGL
* have_cxx was being used incorrectly
* Qt tool detection was outdated
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9046>