pgsql: Reorganize some of the exports list generation code.

Log Message:
-----------
Reorganize some of the exports list generation code.  It seems that this
has been reinvented about four different times throughout history (aix,
cygwin, win32, darwin/linux) and a lot of the concepts are actually shared,
which the code now shows better.

Modified Files:
--------------
    pgsql/src:
        Makefile.shlib (r1.111 -> r1.112)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/Makefile.shlib?r1=1.111&r2=1.112)
    pgsql/src/makefiles:
        Makefile.aix (r1.26 -> r1.27)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/makefiles/Makefile.aix?r1=1.26&r2=1.27)