(3.24) gitlab-ci: Run distcheck, but without actually running the tests

(This change is not applicable to GTK 4, which removed Autotools support.)

This verifies that we can build a release tarball using Autotools, without being subject to known-failing tests (which are less straightforward to fix).

To make this work, enable the Wayland and X11 backends for the Autotools build (distcheck fails when Wayland was not enabled). Enabling one backend (Broadway, in commit 1882ff9b) implicitly disabled all the others, which was probably not intentional, so bring the Autotools build into line with Meson.