PORT: Enable SINGLE_LISTEN_UNSERIALIZED_ACCEPT for AIX 4.3.2

PORT: Enable SINGLE_LISTEN_UNSERIALIZED_ACCEPT for AIX 4.3.2
and above.  Rewrite configure logic for modern levels of
AIX to support future releases of AIX with no changes to
Apache.

This is basically a housekeeping matter to avoid having separate
stanzas for each AIX release and to add some finer detail to
the -DAIX=xxx value so that we know when to enable
SINGLE_LISTEN_UNSERIALIZED_ACCEPT.

I suspect that this fixes the recognition of AIX on ia64, but
nobody complained when it was broken and I don't know where to
find such a beast.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@97091 13f79535-47bb-0310-9956-ffa450edef68