This allows two things
a) One can build with experimental but not have it installed when building
   libphonon software which helps Debian.
b) A config consumer can now specify experimental as COMPONENT and force
   the phonon config to look for experimental and butt out when it can't
   find it. Whereas previously the consumer would have to check the
   variables we set to check whether experimental is even available.

Additionally we continue to quietly look for experimental when it was
built. We do now ignore if it wasn't found thus enabling our builds
to have created experimental but have it not necessarily be available at
buildtime of a consumer.

This is presenting a **slight** incompatibility in that consumers that
did previously not check our variables for whether experimental is
available (i.e. simply assumed it is always available) will now fail iff
the experimental config is not present when building the consumer.
Obviously the consumers did not necessarily build in all setups (i.e. those
without experimental) previously anyway, this is really not much of a
concern however.

built, consumed, exploded