gecko: add native OSS support, replacing ALSA as fallback

OSS is always built but during runtime only selected by default if
neither pulseaudio, jackit or sndio are installed. In particular,
Gnome and KDE users would still be offered PulseAudio by default due
to integration with multi-app volume widgets, current song bars,
screensharing with audio, etc. that prefer to talk over DBus.
Those can opt out via media.cubeb.backend=oss in about:config.

Submitted by: Ka Ho Ng <khng300@gmail.com> (based on)