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