Dynamic linking on AIX

This patch fixes linking on AIX.

Relevant threads on psql-hackers:
http://archives.postgresql.org/pgsql-hackers/2006-09/msg01020.php
http://archives.postgresql.org/pgsql-hackers/2006-09/msg01084.php
http://archives.postgresql.org/pgsql-hackers/2006-09/msg01109.php

Up to now, the default on AIX was to link statically.
This was not intended; it's just that AIX has different
ideas about shared libraries than Linux and nobody cared.

--enable-rpath was matched to the AIX equivalent.

I tested the patch on AIX 5.3 with gcc; it should also
work with IBM's cc. The patch is backward compatible until
AIX 4.3 at least, and I added a kludge to Makefile.shlib so
that it should also build something operational on AIX 3.2.5.

The change to
src/interfaces/ecpg/test/Makefile.regress
is not actually part of the patch, but without it building
fails if you configure --with-ldap --disable-shared

I tested building with --disable-shared and --enable-shared
(with --enable-rpath and --disable-rpath).

Yours,
Laurenz Albe