Fix -pthread link time warnings on OS X by upgrading to
 current ax_pthread.m4 and applying change suggested at
 http://savannah.gnu.org/patch/?8186#comment7

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>
Steve Atkins added to the AUTHORS file

Signed-off-by: Martin Sustrik <sustrik@250bpm.com>