gtype: guard uses of new fundamental type check

g_type_is_fundamentally_a (see bug 730984) is a new API/ABI and is
marked with a version macro.  We should therefore avoid its
unconditional use from G_IS_OBJECT() and G_IS_PARAM_SPEC() which are
APIs that have been around for a long time.

This prevents deprecation warnings from being emitted when these
functions are used with an older GLIB_VERSION_MAX_ALLOWED and also
prevents linking to the new ABI in that case (so that it's possible to
use the resulting binary with an older version of GLib).

https://bugzilla.gnome.org/show_bug.cgi?id=731335