The Fedora base image we use for our CI doesn't always keep Meson up to
date with our requirements, so it's better if we just install Meson via
Python's pip.
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.
