Lower the required Autoconf version to 2.70

Author: akopytov

configure.ac

@@ -1,6 +1,6 @@
 # Process this file with autoconf to produce a configure script.
 
-AC_PREREQ([2.72])
+AC_PREREQ([2.70])
 AC_INIT([sysbench],[1.1.0],[https://github.com/akopytov/sysbench/issues],[sysbench],[https://github.com/akopytov/sysbench])
 AC_CONFIG_AUX_DIR([config])