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>
2 jobs from master in 4 minutes (queued for 17 seconds)
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passed #117536
build-native-autotools

00:03:56

passed #117537
build-native-meson

00:02:41