Fix PGXS vpath build when PostgreSQL is built with vpath
Fix PGXS vpath build when PostgreSQL is built with vpath

PGXS computes srcdir from VPATH, PostgreSQL proper computes VPATH from
srcdir, and doing both results in an error from make.  Conditionalize so
only one of these takes effect.

ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,techdragon/Postgres-XL,adam8157/gpdb,50wu/gpdb,xinzweb/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,greenplum-db/gpdb,ovr/postgres-xl,ovr/postgres-xl,50wu/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,ashwinstar/gpdb,lisakowen/gpdb,50wu/gpdb,zeroae/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,xinzweb/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,adam8157/gpdb,adam8157/gpdb,yazun/postgres-xl,zeroae/postgres-xl,techdragon/Postgres-XL,greenplum-db/gpdb,yazun/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,xinzweb/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,oberstet/postgres-xl,oberstet/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,pavanvd/postgres-xl,adam8157/gpdb,techdragon/Postgres-XL,techdragon/Postgres-XL,ashwinstar/gpdb,greenplum-db/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,lisakowen/gpdb,ovr/postgres-xl,zeroae/postgres-xl,oberstet/postgres-xl,ovr/postgres-xl,greenplum-db/gpdb,jmcatamney/gpdb,pavanvd/postgres-xl,50wu/gpdb,Postgres-XL/Postgres-XL,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb,lisakowen/gpdb,Postgres-XL/Postgres-XL,yazun/postgres-xl,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,xinzweb/gpdb,xinzweb/gpdb,oberstet/postgres-xl,jmcatamney/gpdb,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,zeroae/postgres-xl,ovr/postgres-xl,jmcatamney/gpdb,ashwinstar/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,yazun/postgres-xl,50wu/gpdb