3.24: switch back to Autotools build for GStreamer

It's just too broken now.

 (a) The release tarball doesn't include meson.build. That's a no-go for
     our release infrastructure; we're not going to teach JHBuild to use
     meson for git checkouts but Autotools for tarballs.

 (b) https://bugzilla.gnome.org/show_bug.cgi?id=773867 is a thing,
     requires newer meson to fix, but virtual sysdeps architecture
     doesn't allow for version requirements, and it'd be a year before
     we could safely bump the requirement anyway.

These are fixable problems, but I'm not going to work on them. We'll
revisit in the future.