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. Batckpatch to 8.2.X.

Modified Files:
--------------
pgsql:
configure.in (r1.539 -> r1.540)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.539&r2=1.540)