Use the versioned libmutter*.so versions

Mutter now provides versioned libraries and pkg-config files, meaning
an application using libmutter and friends need to depend on a specific
version of the API.

https://bugzilla.gnome.org/show_bug.cgi?id=777317

js/misc/config.js.in

@@ -15,3 +15,5 @@ const LOCALEDIR = '@datadir@/locale';
 /* other standard directories */
 const LIBEXECDIR = '@libexecdir@';
 const SYSCONFDIR = '@sysconfdir@';
+/* g-i package versions */
+const LIBMUTTER_API_VERSION = '@LIBMUTTER_API_VERSION@'