Install a cleaner solution to the AIX libpq linking problem, as per

Install a cleaner solution to the AIX libpq linking problem, as per
an earlier discussion.  Centralize assumptions about what libpq depends
on in one place in Makefile.global.  I am unconvinced that this list
is complete, but since ecpg seems to have gotten along with just these
entries, we'll try it this way and see what happens.