Rename configure.in to configure.ac

This is standard these days, and aclocal and automake were outputting
warnings about this.

INSTALL

@@ -19,8 +19,8 @@ diffs or instructions to the address given in the `README' so they can
 be considered for the next release.  If at some point `config.cache'
 contains results you don't want to keep, you may remove or edit it.
 
-   The file `configure.in' is used to create `configure' by a program
-called `autoconf'.  You only need `configure.in' if you want to change
+   The file `configure.ac' is used to create `configure' by a program
+called `autoconf'.  You only need `configure.ac' if you want to change
 it or regenerate `configure' using a newer version of `autoconf'.
 
 The simplest way to compile this package is: