fix build with automake 1.13(.1)

AM_CONFIG_HEADER has been deprecated for some time, finally it is removed
on automake 1.13. The attached patch replaces it with AC_CONFIG_HEADERS and
fixes build process with latest automake.

Acked-by: Matthias Andree <matthias.andree@gmx.de>
Message-Id: 20130108093912.7ae7c6f8@leda
URL: http://article.gmane.org/gmane.network.openvpn.devel/7222

Signed-off-by: Gert Doering <gert@greenie.muc.de>
(cherry picked from commit d86d577031577dfd69e5ba104e0ce1cb5192c16a)