Drop autotools build system

This drops all Makefile.am, configure.ac and autogen.sh. It is only a first step and there is probably many places, especially in the doc, that needs to be updated too.

One very encouraging milestone has been achieved by @nirbheek, he managed to build GLib and GStreamer using meson for all their relevant platforms (Windows, Android, iOS, MacOS).

We should still wait for feedback from distros packaging 2.58, I asked @smcv to see if we can have glib+meson in Debian experimental, if it's good for them I don't see any potential blockers any more.

Overall, I'm pretty confident that if any issue is spotted, we can fix it in the 2.59 cycle.