VPATH and DESTDIR support for PL/Perl, using the same techniques employed

VPATH and DESTDIR support for PL/Perl, using the same techniques employed
in interfaces/perl5 a brief while ago.

Also, since building PL/Perl without a shared libperl actually works on
some platforms we can enable it there to get some development happening.
I've only checked off linux right now, but others should be added in the
future.