gnutls: upgrade to 2.12.23

gnutls: upgrade to 2.12.23

Importantly, this fixes CVE-2013-1619.

Upstream doesn't use GNU as a host, so update the SRC_URI.

remove-gets.patch isn't required anymore, obsolete_automake_macros.patch is
merged upstream, and gnutls_fix_for_automake_1.12.1.patch doesn't seem to be
needed.  It was merged and reverted upstream, and my build without it succeeded.

(From OE-Core rev: 9a6395076984350b1dd7005453f97233bbb43132)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>