configure.ac: downgraded autoconf requirements.

Debian buster uses 2.69 and is still much in use.
We also currently use this for our CI.

configure.ac

@@ -1,7 +1,7 @@
 #                                               -*- Autoconf -*-
 # Process this file with autoconf to produce a configure script.
 
-AC_PREREQ([2.71])
+AC_PREREQ([2.69])
 AC_INIT([sane-frontends], [1.1.0], [sane-devel@alioth-lists.debian.net])
 
 AM_INIT_AUTOMAKE