Disable pipe lock for AIX (#4535)
Disable pipe lock for AIX (#4535)

flock function on AIX only permits exclusive lock on file opened
for writing. Exclusive lock on file opened for reading will fail
always. Disable this on AIX platform.
xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,adam8157/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb