Allow static compilation.
Allow static compilation.

We still default to a shared library, but you can use the
"cmake -DBUILD_SHARED_LIBS=off" option to build a static library instead.

50wu/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,xinzweb/gpdb,adam8157/gpdb,jmcatamney/gpdb,50wu/gpdb,greenplum-db/gpos,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gporca,PengJi/gporca-comments,adam8157/gpdb,PengJi/gporca-comments,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,50wu/gpdb,50wu/gpdb,xinzweb/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,jmcatamney/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpos,lisakowen/gpdb,greenplum-db/gpdb,adam8157/gpdb,50wu/gpdb,PengJi/gporca-comments,xinzweb/gpdb,greenplum-db/gpos,jmcatamney/gpdb,greenplum-db/gpdb,PengJi/gporca-comments,adam8157/gpdb,jmcatamney/gpdb,greenplum-db/gporca,xinzweb/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gporca,lisakowen/gpdb,lisakowen/gpdb,greenplum-db/gpos,greenplum-db/gporca,50wu/gpdb,PengJi/gporca-comments,greenplum-db/gpdb