Remove Autotools build system

As discussed in flatpak#5695, I think we're reaching a point where removing
Autotools is preferable to fixing it.

1.14.x continues to use Autotools, so platforms whose Meson version is
too old can stay on that branch until it becomes unsupported. We have
a very conservative Meson dependency (Ubuntu 20.04).

Signed-off-by: Simon McVittie <[email protected]>