Silence -Wunknown-pragmas warnings
Silence -Wunknown-pragmas warnings

Building stream.c has exposed various
> ignoring #pragma omp parallel [-Wunknown-pragmas]
when not linked with openmp.

Be explicit and ask openmp for parallelization only when the binary has been
linked with the library.

Reviewed-by: Jacob Champion <487ac4c1c30bb8aa1aeea1526e59eafc939d7768@pivotal.io>

greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,lisakowen/gpdb,ashwinstar/gpdb,adam8157/gpdb,xinzweb/gpdb,xinzweb/gpdb,jmcatamney/gpdb,adam8157/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,lisakowen/gpdb,ashwinstar/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,ashwinstar/gpdb,xinzweb/gpdb,lisakowen/gpdb,adam8157/gpdb