Take over some debian build settings for SDL
the usual goals: more compatibility while keeping file size small

Add libraries to armabuild for sound compatibility

Enable shared (dynamic runtime) linkage in SDL
with X11, alsa and pulseaudio

pulseaudio in particular pulls in FLAC and ogg as dependencies, blowing
up our appimages. By having SDL load them on demand at runtime, we no
longer have them as hard dependencies.
If none of SDL, alsa or pulse are present on the user's system, they don't
get sound, but they still can play.