build: By default, error out if Opus is missing

Following the commit disabling celt by default, it's quite easy to have
a build without both celt and opus. After this commit, Opus will have to
be installed for a successful build unless one passes --disable-opus.

Signed-off-by: Christophe Fergeau <cfergeau@redhat.com>
Acked-by: Frediano Ziglio <fziglio@redhat.com>