Gtk : Enable wayland support on Linux

[thufschmitt]

Motivation for this change

Gtk3 is currently compiled without wayland support.
This pr enables it by default on Linux, (as it is already done in most other distributions).

I haven't tested it much, except on the packages I use, because I don't have the build power to rebuild the whole dependency closure of gtk3 (but I hope this won't bring much trouble, as the wayland backend is only supposed to be active if the GDK_BACKEND env var is set to wayland).

Note: This triggers quite a huge rebuild, should I submit the pull request to staging instead ?

Things done

• Tested using sandboxing
  (nix.useChroot on NixOS,
  or option build-use-chroot in nix.conf
  on non-NixOS)
• Built on platform(s)
  • NixOS
  • OS X
  • Linux
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

By analyzing the blame information on this pull request, we identified @vcunat, @spwhitt and @urkud to be potential reviewers

[vcunat]

Staged.

[thufschmitt]

Thanks :)

[vcunat]

We originally had it enabled, I think, but after introduction of wayland-protocols the support was silently dropped. But there probably aren't many nix+wayland users, too.

[thufschmitt]

No indeed,
But it's sad, because with a bit of packaging and some appropriate modules, it should be quite usable I think