CI github actions: include compilation in Freebsd

[sarroutbi]

After migrating from Travis to Github actions, it seems no compilation in Freebsd is performed. It would be great to add a pair of additional steps to compile and test in Freebsd operating system.

[hdholm]

It seems like this is hard, bordering on impossible, with github actions. I've seen something about running a FreeBSD VM on top of the mac-os builders, but essentially it isn't a supported architecture by github. So unless github changes something, it would have to be an entirely separate CI system.

[sarroutbi]

Ok, thanks for clarifying @hdholm . If you agree, I will let this issue opened as enhancement just in the case Github actions improves support in FreeBSD support.