Turns out that GCC errors out when building the GLib test suite, as it
now checks for overflows in allocator functions, and we're testing for
those.
This would not be an issue for GTK, but since we're building GLib as a
subproject, we get failures for those as well.
Until we can find out how to disable errors for subprojects, or fix the
GLib test suite not to trip up warnings in GCC, we're going to live
without compiler warnings treated as errors for a while.
xvfb doesn't like C.utf8 and returns XLocaleNotSupported.
While (afaik) C.utf8 and C.UTF.8 should be the same thing, and the former
is returned by locale -a on Fedora, switch to C.UTF-8 to make xvfb happy.
This makes gtk+:gdk tests pass.
The GitLab cache is kept across jobs, whether they succeeded or not:
this means that if a compiler check fails during the Meson
configuration, the small compiler program gets cached and restored the
next time the job is run, thus failing again.
