pmix: fix zlib protection macro usage

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>

opal/mca/pmix/pmix2x/pmix/src/util/compress.c

@@ -1,5 +1,6 @@
 /*
  * Copyright (c) 2016      Intel, Inc. All rights reserved.
+ * Copyright (c) 2017      Cisco Systems, Inc.  All rights reserved.
  * $COPYRIGHT$
  *
  * Additional copyrights may follow
@@ -21,7 +22,7 @@
 #include "src/include/pmix_globals.h"
 #include "src/util/compress.h"
 
-#ifdef HAVE_ZLIB_H
+#if PMIX_HAVE_ZLIB
 bool pmix_util_compress_string(char *instring,
                                uint8_t **outbytes,
                                size_t *nbytes)