AC_CHECK_LIB was not correctly checking for the presence of zlib.
We use PKG_CHECK_MODULES instead to add -lz to libring_la_LDFLAGS.

Change-Id: Icfffb898030f886aae8c2c82dc5327f923f0f78e
Tuleap: #746
[stepan.salenikovich@savoirfairelinux.com: modified to use PKG_CHECK_MODULES and ZLIB_LIBS variable instead of -lz]
Signed-off: Stepan Salenikovich <stepan.salenikovich@savoirfairelinux.com>