Fix compilation on new Mac

[ioan-chera]

I've acquired a new Mac because the old one became unsupported with the latest SDL libraries. Right now I'll need to update the code to work on current Macs. To do list:

• [x] Make sure it builds
• [x] Solve all new pertinent compiler warnings
• [x] Get rid of crutches and stopgaps such as Gulrak Filesystem library
• [x] Update all SDL to the very latest
• [x] Check we don't break Windows and Linux
• [x] Update the expected SDL versions
• [x] Make sure we play music
• [ ] #585

Warning: minimum deployment target moved up to macOS 10.15 (I previously said 10.13, but std::filesystem really requires 10.15).

[ioan-chera]

Comments about the lack of music:

1. The error is in I_SDLRegisterSong here:

   // Try SDL_mixer locally
   rw    = SDL_RWFromMem(data, size);
   music = Mix_LoadMUS_RW(rw, true);

   where music is null and the SDL error is "Unrecognized audio format".

2. Apparently MIDI is a removed feature from the built binaries? https://github.com/libsdl-org/SDL_mixer/issues/419
3. To this end I need to figure how to build a framework locally, or alternatively how to easily bundle all resulting libraries without leaking any dependency to the local system.

Tough, but this is what it takes to have a working new-Mac build.

[ioan-chera]

Temporarily fixed the music problem by using one of the temporary URLs from the linked GitHub issue. I'll need to keep an eye on it when music stops working.

[ioan-chera]

Done for now.