libcanberra is an old abandoned project whose last release
was well over a decade ago, despite getting some few fixes
on its master branch since then, which are unreleased.

My problem with libcanberra is that it can't be built on
pure wayland systems (no X11 headers) which are becoming
more common nowadays.

It is the only thing keeping pavucontrol tied to X11, all
other dependencies (eg gtk+) build fine on pure wayland.

Since canberra is not a core part of the project, let's
make it optional so pavucontrol can build & work on
pure wayland systems.

Ideally in the future libcanberra can be replaced with
something else, however I don't have the bandwidth to
take this on for now, so I'm just making it optional.

Signed-off-by: Adrian Ratiu <adrian.ratiu@collabora.com>