build: Disable celt 0.5.1 by default

This version of the CELT codec has long been obsolete, and Opus support
has been added nearly 5 years ago. It's time we move on and try to stop
using Celt ;)
This commit disables CELT by default, but since this could be an
unexpected change for packagers, if CELT 0.5.1 development headers are
installed, it will error out unless --enable-celt051/--disable-celt051
has been explicitly specified.

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