Require Autoconf >= 2.64

m4_argn macro was introduced in v2.63b-34-g46e87de on the 22nd of Apr 2009.

Change-Id: Id01cc9f0ffdb46cf6dfd26c3bf3b5efa0ddd2984
Reported-by: Franck Jullien <franck.jullien@gmail.com>
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1656


Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>