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.