Add Opus support to CMake builds

[ccawley2011]

This should not be merged until the necessary CMake changes are backported to the SDL branches for opus and opusfile.

[slouken]

Are there pull requests for opus and opusfile?

[ccawley2011]

Are there pull requests for opus and opusfile?

I've opened libsdl-org/opus#1 and libsdl-org/opusfile#1 with the required changes.

[slouken]

Are there pull requests for opus and opusfile?

I've opened libsdl-org/opus#1 and libsdl-org/opusfile#1 with the required changes.

These are in!

[madebr]

I rebased your pr on top of current master to get CI to test with the updated submodules.

[sezero]

After merging this, CI cmake runs are always failing: did we miss something?

[slouken]

@madebr, do you know what's going on here?
https://github.com/libsdl-org/SDL_mixer/runs/6539358047?check_suite_focus=true

[madebr]

The error is in the vendored opusfile:

• its cmake script fails to extract the version and sets 0 to the 2 output variables
• but the caller expects version to be a version, instead of a integer

Due to my workstation currently out of order, I'm unable to push/test a fix.
But I think replacing this set with set(OPUSFILE_PROJECT_VERSION 0.0) or set(OPUSFILE_PROJECT_VERSION 0.1) fixes the error.
It's not perfect because the version is not detected correctly, but it's a quick fix nonetheless.

[sezero]

Or, we can just put a package_version file in there with PACKAGE_VERSION="1.3.1" ?
Possibly to opusfile too, don't know if it's really needed there?

[sezero]

Hm, tried adding the package_version files, still fails
https://github.com/libsdl-org/SDL_mixer/runs/6552834511?check_suite_focus=true

[sezero]

Cmake package_version thing is fixed by this:
libsdl-org/opusfile@9ba5cc3
(maybe good for mainstream submission??)

But build fails for other reasons now - @slouken, for you:
https://github.com/libsdl-org/SDL_mixer/runs/6553099365?check_suite_focus=true#step:7:396

 /usr/bin/ld: CMakeFiles/SDL2_mixer.dir/src/codecs/music_wav.c.o: in function `ParseID3':
music_wav.c:(.text+0x1d4e): undefined reference to `read_id3v2_from_mem'
collect2: error: ld returned 1 exit status

[sezero]

But build fails for other reasons now - @slouken, for you

Well the cmake script was missing mp3utils.c - adding all missing sources fixed it.

[sezero]

I opened a ticked at xiph opusfile: xiph/opusfile#33