Update JSON-GLib dependency

Update JSON-GLib dependency

Currently, Clutter depends on the internal copy of JSON-GLib for
the ClutterScript parser. This is done to allow building Clutter
on platforms that do not have the library installed on the system.

Just like we use the internal PNG/JPEG loader as a fallback in
case we don't have GdkPixbuf or CoreGraphics available, we should
use the internal copy of JSON-GLib only in case the system copy
is not present.

The change is simply to move the default for the --with-json
configure switch from "internal" to "check".

In order to allow stricter compliance, a third setting should
be present: "system", which fails if the system copy is not
available.

We should also change the introspection generation to avoid
breaking in case we require the installed Json-1.0.gir instead
of the generated ClutterJson.gir