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/ui/environment.js

@@ -1,6 +1,8 @@
 // -*- mode: js; js-indent-level: 4; indent-tabs-mode: nil -*-
 
-imports.gi.versions.Clutter = '1.0';
+const Config = imports.misc.config;
+
+imports.gi.versions.Clutter = Config.LIBMUTTER_API_VERSION;
 imports.gi.versions.Gio = '2.0';
 imports.gi.versions.Gdk = '3.0';
 imports.gi.versions.GdkPixbuf = '2.0';