configure: update to and require use of autoconf 2.72 (OSGeo#6533)

Follow up to the de facto update with commit: ec42cfa

Author: nilason

configure.ac

@@ -20,7 +20,7 @@
 # Website for   config.guess, config.sub:
 #               wget http://git.savannah.gnu.org/cgit/config.git/plain/config.guess
 #               wget http://git.savannah.gnu.org/cgit/config.git/plain/config.sub
-#               Note: use autoconf-2.69
+#               Note: use autoconf-2.72
 #############################################################################
 
 define([AC_CACHE_LOAD], )
@@ -31,7 +31,7 @@ $2
 
 AC_INIT
 AC_CONFIG_SRCDIR([configure.ac])
-AC_PREREQ([2.71])
+AC_PREREQ([2.72])
 AC_CONFIG_HEADERS([include/grass/config.h])
 AC_CANONICAL_HOST
 AC_PROG_CC