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.

README.md

@@ -32,10 +32,12 @@ Supported Platforms
 ===================
 - Windows (XP+)
 - Linux
-- BSD (via OSS)
+- BSD
 - Android
+- Emscripten / HTML5
 
-macOS and iOS support is coming soon(ish) via Core Audio.
+macOS and iOS support is coming soon(ish) via Core Audio. Unofficial support is enabled via the OpenAL
+and SDL backends, however I have not tested these personally.
 
 
 Backends
@@ -47,6 +49,7 @@ Backends
 - OSS
 - OpenSL|ES (Android only)
 - OpenAL
+- SDL
 - Null (Silence)