Add configure check for the path to systemd-ask-password

Gentoo Linux installs this in /usr/bin by default. Also, the user may
have installed it in /usr/local/bin if building from source.

Acked-by: David Sommerseth <davids@redhat.com>
Message-Id: 1404419045-14728-1-git-send-email-floppym@gentoo.org
URL: http://article.gmane.org/gmane.network.openvpn.devel/8825

Signed-off-by: David Sommerseth <davids@redhat.com>