configure: Make Wayland backend optional
configure: Make Wayland backend optional

As long as we are not ready to switch over the default backend,
arrange ./configure without explicit backend options makes the
x11 backend mandatory and the wayland backend optional (depending
on whether we find Wayland dependencies).

http://bugzilla.gnome.org/show_bug.cgi?id=709212

davidgumberg/gtk,jadahl/gtk,msteinert/gtk,Lyude/gtk-,davidgumberg/gtk,alexlarsson/gtk,alexlarsson/gtk,alexlarsson/gtk,ahodesuka/gtk,jadahl/gtk,alexlarsson/gtk,msteinert/gtk,alexlarsson/gtk,alexlarsson/gtk,jigpu/gtk,grubersjoe/adwaita,ahodesuka/gtk,jigpu/gtk,davidgumberg/gtk,chergert/gtk,jessevdk/gtk,jessevdk/gtk,jigpu/gtk,grubersjoe/adwaita,Adamovskiy/gtk,davidgumberg/gtk,ahodesuka/gtk,Adamovskiy/gtk,Adamovskiy/gtk,ahodesuka/gtk,alexlarsson/gtk,Lyude/gtk-,jadahl/gtk,Sidnioulz/SandboxGtk,msteinert/gtk,grubersjoe/adwaita,Sidnioulz/SandboxGtk,Lyude/gtk-,jessevdk/gtk,Lyude/gtk-,grubersjoe/adwaita,davidgumberg/gtk,jessevdk/gtk,chergert/gtk,jadahl/gtk,Sidnioulz/SandboxGtk,Sidnioulz/SandboxGtk,jigpu/gtk,jessevdk/gtk,grubersjoe/adwaita,grubersjoe/adwaita,chergert/gtk,msteinert/gtk,grubersjoe/adwaita,chergert/gtk,ahodesuka/gtk,Adamovskiy/gtk,Lyude/gtk-,ahodesuka/gtk,chergert/gtk,Sidnioulz/SandboxGtk,Adamovskiy/gtk,jadahl/gtk,jigpu/gtk,davidgumberg/gtk,jigpu/gtk,jadahl/gtk,msteinert/gtk,Sidnioulz/SandboxGtk,jessevdk/gtk,Adamovskiy/gtk,chergert/gtk,msteinert/gtk,chergert/gtk,jigpu/gtk,jadahl/gtk,Adamovskiy/gtk,jessevdk/gtk,Lyude/gtk-,Lyude/gtk-,grubersjoe/adwaita,Adamovskiy/gtk,jigpu/gtk,davidgumberg/gtk,Lyude/gtk-,alexlarsson/gtk,davidgumberg/gtk,chergert/gtk,ahodesuka/gtk,ahodesuka/gtk,jadahl/gtk