Drop autotools support

Finally. Once this is merged I need to triage anything remaining in the autotools label and delete it.

I did this by deleting the autotools files from git, git greping for their names, and also git grep -iing for the following list of strings and investigating each occurrence:

• Makefile
• autotool
• automake
• autoconf
• GNU make
• libtool
• autogen
• configure
• '\-\-enable\-'
• '\-\-disable\'

po/Makefile.in.in has been deliberately left in place, because it’s installed in /usr/share because it’s copied into projects which use glib-gettextize.

I haven’t dropped the auto* packages from .gitlab-ci/Dockerfile because they’re needed for building the GLib dependencies on the Android CI machine.

The changes to the --{enable,disable}-debug configure option are non-trivial and might be worth a follow-up issue if anyone cares. I’m happy with them in their state at the head of this branch.