Add GH Actions CI for debug builds

[Miepee]

Currently this gets triggered on

• new push on master
• a PR that targets master
• when manually triggered from the actions menu from someone with perms.

All OS build their GUI and CLI without any optimizations.
Linux uses g++ to build. macos uses a modified clang++ to build in order to get around -fopenmp erroring. Windows uses g++ because MSVC throws errors and warnings out like crazy ( See https://github.com/Miepee/Flips/actions/runs/9196713315/job/25295435231 if you want to see the full log of those )

Macos currently on purpose targets macos-13 instead of macos-latest, as macos-latest is ARM-only, while macos-13 is stil using x64_86 which means it'll run on both intel macs and m1 macs via rosetta.

Helps addressing more things from #56

[Alcaro]

Wow, that is one impressive wall of stupid MSVC warnings. I'm not even remotely interested in Spectre mitigations and unreferenced parameters. I guess MSVC treats -Wall like Clang's -Weverything - enable every single warning including the stupid ones.

The #include errors look legitimate, I'll fix that. Flips isn't supposed to use anything other than standard C++, but autoconf likes creating platform-dependent config.h files. (Fair chance there's some other error hiding behind that, though; I don't trust anything that throws a dozen 'ignoring unknown option' warnings. I'm surprised MSVC got that far on a GCC command line.)

I doubt there's any need to build GTK on mac, or CLI on more than one platform, but good enough. Better too much than too little.

This feels like the kind of thing I'd like to test a bit more, but I'm not sure how to test before merging, so whatever #yolo

[Alcaro]

I thought it'd need to enable gh-actions by installing some app in some config panel, but it seems to be enabled just by creating that file.

Let's turn off Appveyor then.