Polish the SDL backend and the Emscripten build.

* Remove dependency on SDL.h where possible
 * Use run-time linking where possible
 * Support both SDL 1.2 and SDL 2
 * Add device enumeration when using SDL 2

The Emscripten build currently uses SDL 1.2 because it avoids the need
for "-s USE_SDL=2" when compiling. This may change later if there's
ever a need or if there's enough demand.

examples/advanced_config.c

@@ -54,6 +54,7 @@ int main(int argc, char** argv)
         mal_backend_oss,
         mal_backend_opensl,
         mal_backend_openal,
+        mal_backend_sdl,
         mal_backend_null    // Lowest priority.
     };