libxml2: Fix zlib detection the right way

The previous fix (#557) forced -lz into LDFLAGS instead of fixing the
real issue where configure was failing to detect zlib in the first
place. This was happening because it was looking in /lib, resulting in
conflicts with the host libraries.

Signed-off-by: James Le Cuirot <james.le-cuirot@yakara.com>
Merge pull request #585 from chewi/master

libxml2: Fix zlib detection the right way