BuildSystem: Raise autoconf version requirement to 2.60a.

AC_COMPUTE_INT requires at least autoconf 2.60a.

This affects only those who contribute to Asterisk, only those who had to use
the script ./bootstrap.sh. Furthermore, this change just makes sure nobody is
using a too old autoconf.

ASTERISK-16951

Change-Id: Ibca850e2fe0e77d935207bd959bacf7197d7f637

configure.ac

-# Process this file with autoconf to produce a configure script.
-#
-# Make sure we use autoconf 2.60 to generate the "configure" script,
-# in case we want to commit it. Other than that, version 2.59 is
-# perfectly fine for our purposes, so people who want to modify
-# this file just have to remember to set the AC_PREREQ argument
-# to something that suits their needs.
-
-AC_PREREQ(2.60)
+AC_PREREQ(2.60a)
 
 AC_INIT([asterisk], [trunk], [https://issues.asterisk.org])