It's possible that we can have zlib.h but still not have zlib support.
Use the correct macro to protect the usage of calling zlib functions.
This fixes 32-bit MTT builds at Cisco (e.g.,
https://mtt.open-mpi.org/index.php?do_redir=2389).
Submitted upstream to PMIX: https://github.com/pmix/master/pull/290
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>