Build: Let Autoconf check its own minimum version.
After invoking `autoconf', Autoconf will check to see if it is older
than the minimum required version.
Right now, we require Autoconf 2.60, which is specified at the top of
configure.in:
AC_PREREQ(2.60)
If Autoconf < 2.60 is installed, an error message is produced:
$ ./autogen.sh
processing .
Running autoconf ...
configure.in:4: error: Autoconf version 2.60 or higher is required
configure.in:4: the top level
autom4te: /usr/bin/m4 failed with exit status: 63