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.

Tags:
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REL8_1_STABLE

Modified Files:
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pgsql:
configure.in (r1.431.2.20 -> r1.431.2.21)
(http://developer.postgresql.org/cvsweb.cgi/pgsql/configure.in?r1=1.431.2.20&r2=1.431.2.21)