include libmusicbrainz usage example (web_query)
don't fail if libmusicbrainz isn't found

We only need the library for an example
and don't actually require it.
So be quiet about searching it.
build disc_metadata also with autotools

The key is that libmusicbrainz5 should *not* be added to LIBS,
because that would link libdiscid to it!
Merge pull request #1 from JonnyJD/libmusicbrainz

LIB-54: Add libmusicbriainz / query metadata example