[Delta] Managing GStreamer and dependencies

[Remko Tronçon]

> The introduction of GStreamer as a dependency complicates things.  Psi is
> generally known as being easy to build.  If you were even slightly
> inconvenienced by something like QCA, then GStreamer will be a living hell.

It's obvious that Psi should be able to compile without gstreamer. If
possible, it would be great if we could compile in gstreamer the way
we do with QCA (by simply dropping the source inside the psi source
tree), but we'll have to see whether that's feasible later.

For now, I would leave it as compile time check whether gstreamer
support can be enabled, and look at making this pluggable or whatnot
later. This is not something that has any impact on the implementation
anyway.

cheers,
Remko