Remove the internal copy of JSON-GLib

The internal copy of JSON-GLib was meant to go away right after the 1.0
release, given that JSON-GLib was still young and relatively unknown.

Nowadays, many projects started depending on this little library, and
distributions ship it and keep it up to date.

Keeping a copy of JSON-GLib means keeping it up to date; unfortunately,
this would also imply updating the code not just for the API but for the
internal implementations.

Starting with the 1.2 release, Clutter preferably dependend on the
system copy; with the 1.4 release we stopped falling back automatically.
The 1.6 cycle finally removes the internal copy and requires a copy of
JSON-GLib installed on the target system in order to compile Clutter.

clutter/clutter-script-private.h

@@ -27,7 +27,8 @@
 #define __CLUTTER_SCRIPT_PRIVATE_H__
 
 #include <glib-object.h>
-#include "clutter-json.h"
+#include <json-glib/json-glib.h>
+
 #include "clutter-color.h"
 #include "clutter-types.h"
 #include "clutter-script.h"