Use native strtoull() with MSVC 2013.

MSVC 2013 C library now defines strtoull() function,
so use the native implementation when available.

Signed-off-by: James Yonan <james@openvpn.net>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1398064204-26476-3-git-send-email-james@openvpn.net>
URL: http://article.gmane.org/gmane.network.openvpn.devel/8561
Signed-off-by: Gert Doering <gert@greenie.muc.de>