Simplify CI by using a build matrix

[Fohlen]

Your GitHub actions workflows could be boiled down to two configs:

• autotools with clang / gcc
• cmake

by using matrix builds.
Unfortunately I don't use this repo as much / don't have time to contribute such a build matrix, but wanted to make you aware that it exists.
All the best

[murraycu]

I'm aware of it, yes, thanks, and I've used it elsewhere. I vaguely remember that it wasn't actually possible to make that work but I'd gladly accept a change that does this.