configure: require opt-in to autotools

Autotools is going away. Break the autotools build so that people are
guaranteed to notice before it is gone. If they have problems with Meson, they
can still use --enable-autotools to build with autotools, but we really want to
hear about any problems.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>