gnutls: fix libz autodetection failure

The configure script finds libz in the distribution libraries, which causes
zlib support to be dropped from the cross-compiled GnuTLS.

Signed-off-by: Antoine Pierlot-Garcin <antoine@bokbox.com>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>