Add Msys2 and CI support

[arsdragonfly]

also adds logging to stderr.txt and stdout.txt, which fixes(?) #268 and #130 .

[NotAFile]

to test all the thousands of commits this repo gets a day

[rakiru]

If you want to try continuing this project yourself, it might be a better idea to fork it and work on it there. I don't think any of the existing team has an interest in working on iceball at the moment, so PRs are unlikely to get merged, at least in a reasonable time frame.

[iamgreaser]

Eh, on one hand, good point, on the other hand, now with AoS 1.x being shut down there may be renewed interest in this.

LGTM. I'd be down for merging it unless anyone objects.

[rakiru]

If you're gonna set up CI, I'd suggest making it actually upload releases, since there's next to zero point in just building this (no tests, etc.). Perhaps if it built on Linux and Windows, then it would at least work to test that it works on gcc and VC++. There was a CircleCI config kicking around somewhere (might have been me, not sure) that does Windows and Mac builds, and uploads them as GitHub releases.

Either way, no real objection.

[arsdragonfly]

@rakiru much of that has been done here including building on Windows w/ different toolchains & producing releases, but the CMakeLists.txt has been completely rewritten (sourceball is no longer required since dependencies are fetched and built using Hunter, and sackit is added to the source tree). I'll do some cleanup and add them to the changeset of this PR if you don't mind.

[arsdragonfly]

Msys2 support and Appveyor CI have been added. Crude logging to stderr.txt and stdout.txt has been set up in main.c, and -mwindows should get rid of the console window on non-Msys2 builds, which kinda fixes #268 and #130 .

[arsdragonfly]

One thing that sucks for Appveyor is that you'd have to add the build status badge later into README.md manually after you've connected your repo to the CI, since the SVG URL is some random crap.