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).