pkglibdir versus libdir?

Peter, I'm a little confused about the difference between the libdir and
pkglibdir settings created by configure.  What's supposed to go where?

I got a complaint from a Red Hat person that the regression tests failed
on a 64-bit machine.  Some investigation revealed that the problem was
that plpgsql.so got installed into $libdir while pg_regress.sh expected
to find it in $pkglibdir.  (The fact that they were different was an
RPM configuration bug, but if we are going to have two variables we
certainly ought to support their being different...)  I don't know how
to decide which one is wrong.
        regards, tom lane