Add multi-threading variants to sdl2_mixer (emscripten-core#23094)

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
(emscripten-core#22946) changes.

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

tools/ports/sdl2_mixer.py

@@ -10,8 +10,10 @@
 
 deps = ['sdl2']
 variants = {
-  'sdl2_mixer_mp3': {'SDL2_MIXER_FORMATS': ["mp3"]},
-  'sdl2_mixer_none': {'SDL2_MIXER_FORMATS': []},
+  'sdl2_mixer-mp3': {'SDL2_MIXER_FORMATS': ['mp3']},
+  'sdl2_mixer-none': {'SDL2_MIXER_FORMATS': []},
+  'sdl2_mixer-mp3-mt': {'SDL2_MIXER_FORMATS': ['mp3'], 'PTHREADS': 1},
+  'sdl2_mixer-none-mt': {'SDL2_MIXER_FORMATS': [], 'PTHREADS': 1},
 }
 
 
@@ -25,7 +27,9 @@ def get_lib_name(settings):
 
   libname = 'libSDL2_mixer'
   if formats != '':
-    libname += '_' + formats
+    libname += '-' + formats
+  if settings.PTHREADS:
+    libname += '-mt'
   libname += '.a'
 
   return libname
@@ -68,6 +72,9 @@ def create(final):
         '-DMUSIC_MID_TIMIDITY',
       ]
 
+    if settings.PTHREADS:
+      flags.append('-pthread')
+
     build_dir = ports.clear_project_build('sdl2_mixer')
     include_path = os.path.join(source_path, 'include')
     includes = [