PR: Clang Linux build, appimage, source builds

[mcarans]

I've made a PR: https://github.com/OoliteProject/oolite ... 32/changes

Packages corresponding to that PR can be found here: https://github.com/mcarans/oolite/actio ... 0256287244. I have tested on Windows and Linux (Ubuntu).

The PR fixes the broken Linux build. It creates AppImages for Linux based on the more performant Clang build (which is also recommended by GNUStep devs) with espeak-ng. It also adds a GH Actions workflow to test source builds on Fedora and Arch. (Ubuntu is tested by building the AppImage). That workflow could one day be expanded to run tests on more platforms.

Once this PR is merged, I can look at making another PR to create modern Windows packages in the release instead of legacy whenever we're ready for that.