Add FreeBSD CI using Cirrus CI.

[arrowd]

Testing is omitted for the moment because AsMonitor tests fail due to FreeBSD-specific bug in glib: https://gitlab.gnome.org/GNOME/glib/-/issues/576

I might also miss some dependencies, so the commit might need amending after the repository enables Cirrus CI.

Obsoletes #485 PR

[arrowd]

Bump, can we get this in?

[ximion]

To make this a bit more maintainable, would it be possible to make Cirrus call tests/ci/run-build.sh and use different build options of *BSD? Or does FreeBSD not support /etc/os-release, so adding support for it would be more tedious in that file? (it would be nice to configure all CI builds in one place...)

[arrowd]

Sure, it seems we can reuse that.

[arrowd]

I see you changed this line 2 weeks ago. I guess, no it shouldn't be needed now.

[ximion]

Good! I guess this will not actually build before I merge it, so let's merge it now and hopefully it will be fine (I will probably need you to address any non-obvious issues, as I am less familiar with FreeBSD).

Thank you for the patch and sorry for this taking so long!

[ximion]

Looks like that build environment has no C compiler. Can you please fix that?