build: Allow disabling X11

This patch is a proposal to have a working Wayland only builds without blocking on #3363.

We still also need to do two things to fix some gobject introspection warnings in Wayland-only builds but they are not blockers per-see.

• Investigate if we can remove the meta_window_x11_get_group / meta_group_list_windows usages from GNOME Shell as they are X11 only APIs

../src/meta/group.h:37: Warning: Meta: meta_group_list_windows: return value: Missing (element-type) annotation
../src/meta/group.h:34: Warning: Meta: meta_window_x11_get_group: return value: Invalid non-constant return of bare structure or union; register as boxed type, add (copy-func) and (free-func), or (skip)

• Possibly revert eb1d1883 ../src/meta/meta-plugin.h:41: Warning: Meta: symbol='XEvent': Skipping foreign identifier 'XEvent' from namespace xfixes

Depends on:

• !3254 (merged)
• !3607 (merged)
• !3765 (merged)

Relevant PRs:

• gnome-shell!3362
• gnome-kiosk!41

Closes #2272