help wanted adding pthread support to sdl2_mixer

Hi,

I wanted to add pthread support to the sdl2_mixer port just like sdl2_ttf and sdl2_image have it.
I am relatively new to python and emscripten so I got a question.

how is this `variants` being used, is there some documentation somewhere ?
in sdl2_mixer for example are two variants:

```
variants = {
'sdl2_mixer_mp3': {'SDL2_MIXER_FORMATS': ["mp3"]},
'sdl2_mixer_none': {'SDL2_MIXER_FORMATS': []},
}
```

where is the decision being made that sdl2_mixer_mp3 is being used ?

in the `__init__.py` I can see that there is the port_variants variable where the variants gets collected into.
And in the `embuilder.py`, the function `get_port_variant` is using it.

But I still don't understand it fully... what is calling "`get_port_variant`" ? would a name like "`sdl2_image-png-mt`" or equivalent then `sdl2_mixer_mp3-mt` then being build somewhere ?