Build Wayland backend by default

Unless an explicit backend is enabled via configure option, build
both the X11 and Wayland backends.

https://bugzilla.gnome.org/show_bug.cgi?id=705498

configure.ac

@@ -314,6 +314,7 @@ if test -z "$backend_set"; then
     enable_win32_backend=yes
   else
     enable_x11_backend=yes
+    enable_wayland_backend=yes
   fi
 fi