build-sys: deprecate the pulseaudio backend

The PulseAudio audio backend is still the default. However, it will be
deprecated in favour of the GStreamer backend in a future release.

Then PulseAudio will be disabled by default, and later on removed.

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Victor Toso <victortoso@redhat.com>