Add multi-threading variants to sdl2_mixer

[db-tech]

I added support for multi threading (-pthread) to the sdl2_mixer port.
For me this was kind of new but I did try to follow the prinziples of the sdl2_image adn sdl2_ttf (#22946) changes.

I also changed the "_" to "-" for consistency.

Let me know if there is something wrong or missing.

[sbc100]

Seems reasonable to me. Does SDL2_mixer actually need thread support?

[db-tech]

Should fix some dependency issues discussed here: #23093

To summerize:
Without the changes I get errors like:

AssertionError: attempt to lock the cache while a parent process is holding the lock (sysroot/lib/wasm32-emscripten/libSDL2.a)

So It might be a dependency problem because mixer depends on another SDL2 version (-mt) then the other SDL2 libs and would differ from the SDL2 lib that is used when -pthread is enabled.

[ololoken]

lgtm
tested and fix works

[sbc100]

Can you use single quote here and above.

[sbc100]

It would be good to a test here that depends on this new variant. What is the set of command line flags you used to cause the AssertionError: attempt to lock the cache while a parent process is holding the lock (sysroot/lib/wasm32-emscripten/libSDL2.a) failure?

[sbc100]

Also can you rebase or merge so you are up-to-date with main?

[oleg-derevenetz]

Hi @sbc100

What is the set of command line flags you used to cause the AssertionError: attempt to lock the cache while a parent process is holding the lock (sysroot/lib/wasm32-emscripten/libSDL2.a) failure?

Here is a reproducible example of the build failure when building the project that depends on sdl2-mixer with -pthread:

https://github.com/oleg-derevenetz/fheroes2/actions/runs/12609145320/job/35142382904#step:5:40

and here is the corresponding test PR:

oleg-derevenetz/fheroes2#218

If there are any issues with maintaining this PR, I can create my own (based on this one with credits to @db-tech) to speed up the process, because I'm interested in solving this build issue, since I would like to build my project for WASM with threads... eventually.

[db-tech]

Hi, sorry for not answering.
I am currently on vacation and usually off the grid.

Tomorrow, I might have time to address the comments.

Otherwise, I am back next week.

[db-tech]

As @oleg-derevenetz said "-pthread" is the option which results in issues.
I changed the double quotes to single ones and, fetched upstream and rebased, I hope that was correct.
Otherwise, let me know.