gcc 7.[1,2] (at least) fails to correctly parse the OSX 10.13 sys/syslog.h
header. As a results we need to potect syslog support in OPAL, PMIX and
ORTE.
Signed-off-by: George Bosilca <bosilca@icl.utk.edu>
Signed-off-by: Jeff Squyres <jsquyres@cisco.com>