Search zlib without pkg-config

NetBSD has zlib installed in base system and no .pc file for it. In
order to discover it, fallback to AC_CHECK_LIB is PKG_CHECK_MODULES
fail to find it.

This enables cdc xlator, and we pass tests/basic/cdc.t

BUG: 764655
Change-Id: Ide84402aa38edc2709d12e2530401c6b8c722529
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/8437
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Harshavardhana <harsha@harshavardhana.net>
Tested-by: Harshavardhana <harsha@harshavardhana.net>