Revert "Update autoconf to new version, to silence noise-warnings"

This reverts commit f3e57c1.

Author: marta-lokhova

configure.ac

@@ -5,7 +5,7 @@
 # under the Apache License, Version 2.0. See the COPYING file at the root
 # of this distribution or at http://www.apache.org/licenses/LICENSE-2.0
 
-AC_PREREQ([2.71])
+AC_PREREQ([2.68])
 AC_INIT([stellar-core],[0.1],[],[],[http://www.stellar.org])
 # tar-ustar is required for long file names when libsodium is bundled
 AM_INIT_AUTOMAKE([-Wall -Wextra -Wconversion subdir-objects tar-ustar silent-rules])