Fix build with Meson 0.48.0 and 0.48.1

This works around a Meson bug which will be fixed in the next release. The workaround seems harmless to me.

I also adjusted the GitLab CI configuration to build against Fedora Rawhide, rather than hardcoding Fedora 28. This would have caught this problem sooner.

We should probably include this in the next 3.30 point release.