openal-soft: Add PulseAudio backend, make non-OSS backends work.

- Added PulseAudio backend support.
- Made ALSA and PulseAudio backends optional.
- Made ALSA and PulseAudio backends actually work by adding to NIX_LDFLAGS (because package uses dlopen).

TEST: Tested all 3 backends (OSS, ALSA, PulseAudio) using openal-info and my upcoming Warzone 2100 update, by selecting the backend in .alsoftrc. It also works without .alsoftrc (I think it first tries pulse then alsa then oss).