Guard against using plperl's Makefile without specifying --with-perl.
Guard against using plperl's Makefile without specifying --with-perl.

The $(PERL) macro will be set by configure if it finds perl at all,
but $(perl_privlibexp) isn't configured unless you said --with-perl.
This results in confusing error messages if someone cd's into
src/pl/plperl and tries to build there despite the configure omission,
as reported by Tomas Vondra in bug #6198.  Add simple checks to
provide a more useful report, while not disabling other use of the
makefile such as "make clean".

Back-patch to 9.0, which is as far as the patch applies easily.

adam8157/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,zeroae/postgres-xl,tpostgres-projects/tPostgres,lisakowen/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,50wu/gpdb,pavanvd/postgres-xl,greenplum-db/gpdb,Postgres-XL/Postgres-XL,yazun/postgres-xl,50wu/gpdb,arcivanov/postgres-xl,techdragon/Postgres-XL,jmcatamney/gpdb,arcivanov/postgres-xl,lisakowen/gpdb,postmind-net/postgres-xl,adam8157/gpdb,ovr/postgres-xl,zeroae/postgres-xl,arcivanov/postgres-xl,kmjungersen/PostgresXL,adam8157/gpdb,arcivanov/postgres-xl,jmcatamney/gpdb,xinzweb/gpdb,lisakowen/gpdb,ashwinstar/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,50wu/gpdb,adam8157/gpdb,tpostgres-projects/tPostgres,jmcatamney/gpdb,pavanvd/postgres-xl,adam8157/gpdb,ashwinstar/gpdb,kmjungersen/PostgresXL,greenplum-db/gpdb,ashwinstar/gpdb,snaga/postgres-xl,tpostgres-projects/tPostgres,jmcatamney/gpdb,ovr/postgres-xl,tpostgres-projects/tPostgres,lisakowen/gpdb,snaga/postgres-xl,jmcatamney/gpdb,xinzweb/gpdb,ashwinstar/gpdb,zeroae/postgres-xl,jmcatamney/gpdb,50wu/gpdb,xinzweb/gpdb,pavanvd/postgres-xl,kmjungersen/PostgresXL,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,postmind-net/postgres-xl,kmjungersen/PostgresXL,ashwinstar/gpdb,zeroae/postgres-xl,Postgres-XL/Postgres-XL,ashwinstar/gpdb,techdragon/Postgres-XL,arcivanov/postgres-xl,50wu/gpdb,snaga/postgres-xl,greenplum-db/gpdb,Postgres-XL/Postgres-XL,oberstet/postgres-xl,50wu/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,ovr/postgres-xl,ashwinstar/gpdb,greenplum-db/gpdb,lisakowen/gpdb,yazun/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,arcivanov/postgres-xl,zeroae/postgres-xl,greenplum-db/gpdb,oberstet/postgres-xl,50wu/gpdb,greenplum-db/gpdb,techdragon/Postgres-XL,Postgres-XL/Postgres-XL,techdragon/Postgres-XL,ovr/postgres-xl,yazun/postgres-xl,tpostgres-projects/tPostgres,adam8157/gpdb,ovr/postgres-xl,yazun/postgres-xl,jmcatamney/gpdb,oberstet/postgres-xl,lisakowen/gpdb,postmind-net/postgres-xl,adam8157/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,postmind-net/postgres-xl,kmjungersen/PostgresXL,snaga/postgres-xl,xinzweb/gpdb,postmind-net/postgres-xl,oberstet/postgres-xl,snaga/postgres-xl