Fix wrong configure.ac parsing of --enable-async-push

Fix wrong configure.ac parsing of --enable-async-push

AC_ARG_ENABLE() was used wrong, which led enable_async_push to
always be set, regardless if --enable-async-push or --disable-async-push
was used.

Also spotted the exact same patch when writing this commit message as
GitHub PR#70.

Trac: #786
Signed-off-by: David Sommerseth <davids@openvpn.net>
Acked-by: Lev Stipakov <lstipakov@gmail.com>
Message-Id: <1481062251-18349-1-git-send-email-davids@openvpn.net>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13411.html