[packagekit] configure needs --with-default-backend and --backend

[Ken VanDine]

I noticed that we now need both --with-default-backend and --backend
or only the dummy backend gets built.  I attempted to build with
--with-default-backend=conary and it only built the dummy backend.  If
we want this behavior, we should move the tests for /usr/bin/whatever
to the --with-default check instead of the default one.

--Ken