pgsql: Provide a more helpful error message when there is an autoconf

Log Message:
-----------
Provide a more helpful error message when there is an autoconf version
mismatch; backpatch.

Modified Files:
--------------
pgsql:
configure.in (r1.540 -> r1.541)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.540&r2=1.541)