renaming configure.in to configure.ac

It was mentioned elsewhere in passing that a new Autoconf release might 
be coming.  That one will warn about the old naming "configure.in" and 
request "configure.ac".  So we might want to rename that sometime. 
Before we get into the specifics, I suggest that all interested parties 
check whether buildfarm scripts, packaging scripts, etc. need to be 
adjusted for the newer name.

-- 
Peter Eisentraut              http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services