Build: Let Autoconf check its own minimum version.

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