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

Log Message:
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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.

Modified Files:
--------------
    pgsql/contrib/dblink:
        Makefile (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/contrib/dblink/Makefile.diff?r1=1.12&r2=1.13)
    pgsql/src:
        Makefile.global.in (r1.227 -> r1.228)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/Makefile.global.in.diff?r1=1.227&r2=1.228)
    pgsql/src/interfaces/ecpg/compatlib:
        Makefile (r1.24 -> r1.25)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/compatlib/Makefile.diff?r1=1.24&r2=1.25)
    pgsql/src/interfaces/ecpg/ecpglib:
        Makefile (r1.41 -> r1.42)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/interfaces/ecpg/ecpglib/Makefile.diff?r1=1.41&r2=1.42)