Conditional availability of properties

[Arun Raghavan]

Hey folks,
I'm looking at adding the GstStreamVolume interface to pulsesrc since
PulseAudio 1.0 will support per-source-output volumes. However, there is
one potential problem here that I believe might raise objections.

The interface (which is really just the "volume" and "mute" properties)
will only be available if pulsesrc is compiled against PulseAudio 1.0
since we want to be able to compile against older versions as well.

IMO this isn't a huge problem if we document that users should be
checking to see if pulsesrc implements the interface before using it
(this is what I've seen users of pulsesink do as well).

Does this sound okay?

-- Arun