17899dc9b6
[wl_shell] is officially [deprecated], so provide support for the
XDG-shell protocol should be provided by all desktop-like compositors.
(In case they don't, we can of course fall back to wl_shell).
Note that the [XML spec] is provided by the `wayland-protocols`
git repository, which is provided by the Wayland project.
[wl_shell]: https://people.freedesktop.org/~whot/wayland-doxygen/wayland/Client/group__iface__wl__shell.html
[deprecated]: 698dde1958
[XML spec]: https://github.com/wayland-project/wayland-protocols/blob/master/stable/xdg-shell/xdg-shell.xml
All aclocal .m4 files we need are put here and cat'd to acinclude.m4 in the source root. Official ones (taken from the relevant devel packages) are named as-is, unofficial ones (or changed ones) get a gst-prefix.