Add mingw-w64 support

[programmerjake]

I got it to cross compile from Debian 12 using Autotools.

I ended up deciding to just disable UDP support on Windows, since I don't need it and it's a large amount of porting work (I'm not familiar with how Windows does multicast or finding interfaces).

Since mingw-w64 doesn't have a <regex.h>, I replaced all the regexps with C code that should have equivalent/improved functionality.

I also added .clang-format with a style that matched what existed before relatively closely (I couldn't match the line break in between else and if) and formatted all the code.

Because the code was reformatted, you'll definitely want to review it commit-by-commit rather than just looking at the combined diff.

Please don't squash or rebase that commit unless you also update .git-blame-ignore-revs.

I added CI that runs on GitHub Actions, it tests building on ubuntu 22.04 and 24.04 with either meson or autotools as well as on windows in msys2 with autotools. It works on my fork on Linux and Windows, but you seem to not have CI enabled on your repository, so it doesn't show up in this PR.