Build Emulator in CI for Windows, Linux and Mac

[maxgerhardt]

Adds a Github Actions workflow file that build the x16emu binary on

• Windows using mingw64
• Linux using Ubuntu 20.04 (ubuntu-latest)
• Mac on Mac OS 11 Big Sur

Automatically packages in the latest ROM build (rom.bin, all .sym files) from the x16-rom repository's Github actions in the uploaded artefact.

Thus enables easy download of precompiled binaries for each emulator commit and continuous testing for compilability on every commit.

The ROM repo is currently pointing at my fork, please merge https://github.com/commanderx16/x16-rom/pull/360 first then the repo can be changed to the official commanderx16 one.

Contains minimum changes to the Makefile with "set variable only if variable wasn't set from the outside" changes so that it picks up the correct sdl2-config program on MinGW and a configurable path for the static SDL2 library on Mac. Existing build processes should thus not be broken.

PS: Please squash upon merge, you don't want 20 commits from me)

Reference run: https://github.com/maxgerhardt/x16-emulator/actions/runs/3397596494

[indigodarkwolf]

And the other cool stuff, just as worthy as https://github.com/commanderx16/x16-rom/pull/360. And also just as likely I'll squash-and-merge this by the end of the weekend unless @mist64 objects. :3

[maxgerhardt]

We're gonna have to make a follow-up PR now, the ROM merge was supposed to be done first, because then the

gh run download -R maxgerhardt/x16-rom -n "ROM Image" --dir latest_rom

Commands can be redirected to the commanderx16/x16-rom repo, with no dependencies on me.

[indigodarkwolf]

Doh. Well, at the very least, it's confirming that the build succeeds.