Add verification pipelines for more platforms and compilers

[christianhujer]

@schmonz had problems testing AceUnit on NetBSD/arm64 with gcc10.
We should test more platforms and compilers automatically on GitHub.
For whatever GitHub doesn't have, we should setup a build farm with self-hosted runners.

[schmonz]

I happen to have a half-assed project in progress about this: I picked a very simple pkgsrc package and have been trying to get it building on as many GitHub-available platforms as possible. Endgame is probably extracting the generic building-on-a-platform glue to its own GitHub Action. In the meantime, ignore/avoid the complexity and just take note of whichever platforms look useful to you.