* Glib.exports
	* Glib.xs
	* gperl.h: Add gperl_sv_defined, which checks an SV for
	definedness just like Perl's defined() does.

	* GBoxed.xs
	* GError.xs
	* GObject.xs
	* GSignal.xs
	* GType.xs
	* GValue.xs
	* Glib.xs: Use gperl_sv_defined instead of SvOK to check SVs for
	definedness.  This keeps Glib from erroneously thinking your
	scalars are undefined, for example when using tied hashes.

	* MANIFEST
	* t/tied_definedness.t: Test that values from tied hashes get
	through the definedness checks.

svn path=/trunk/; revision=919