Add MYSYS2 example to Windows compilation instructions, update MinGW instructions

[oblivioncth]

This build procedure overall requires less work and avoids a bug with MinGW's SDL2mixer package as detailed here: https://gitlab.com/Tapani/1oom/-/issues/68

Additionally, the build uses a package of SDL2_mixer that has fluidsynth support available, while the copy of SDL2_mixer acquired in the original instructions did not. This way, users can utilize their own soundfonts with the binaries that this procedure produces unlike before.

Not sure if fluidsynth support is enabled in artifacts added to the releases of this repository, though since the fluidsynth dll isn't included it won't work either way (unless you're linking it statically).

I didn't know if you would prefer this to be merged into a different branch, like core perhaps, but I assumed master was appropriate.

Not sure if/when I'll have the time, but eventually I'd like to follow this up with adding CMake build scripts (at least for Windows to start) to the repository.

[oblivioncth]

• Added the workaround for the MinGW SDL_mixer pkgconfig bug.
• Heavily simplified the MSYS2 build example

The instructions should still be clear given the information provided throughout the rest of the file.

[oblivioncth]

Just specifically state which MinGW packages are needed.

[oblivioncth]

Update links to SDL2/SDL_mixer MinGW development libraries in MinGW instructions.

[1oom-fork]

Partially merged https://github.com/1oom-fork/1oom/commit/ecd43347fb087b6f097a2c9ce924ec0e69cea6b8

[1oom-fork]

Fixed static build e3cfc03885792d35e3bfe56d2f78954d84f5fe55

[1oom-fork]

The MSYS2 guide is done. Thank you for your efforts. https://github.com/1oom-fork/1oom/commit/8c853c381486007d29e161721a9c6a8507aa7efd