build: Remove autotools

To avoid the burden of maintaining multiple build systems, autotools
support has been removed.

GitLab CI configuration has also been updated to use meson.
7 jobs for remove-autotools in 10 minutes and 50 seconds (queued for 4 seconds)
latest
Status Job ID Name Coverage
  Build
passed #248589
flatpak

00:10:20

manual #248588
allowed to fail manual
gtk-doc
passed #248585
master

00:01:16

manual #248586
allowed to fail manual
meson
manual #248587
allowed to fail manual
meson-gtk-doc
 
  Deploy
passed #248590
deploy

00:00:29

manual #248591
allowed to fail manual
stop_deploy