Disable certificate notBefore/notAfter sanity check on OpenSSL < 1.0.2

Disable certificate notBefore/notAfter sanity check on OpenSSL < 1.0.2

The SSL_CTX_get0_certificate() function I used in 091edd8e is available in
OpenSSL 1.0.2+ only.  Older versions seem to not have a useful alternative.
The remaining option would then be to create a cache for our parsed
certificate, but that would mean adding more struct members and code for
the select group of people that do use an up-to-date openvpn, but do not
update their openssl.  I don't think that's worth it.  So just disable the
code for older openssl versions.

Signed-off-by: Steffan Karger <steffan@karger.me>
Acked-by: Gert Doering <gert@greenie.muc.de>
Message-Id: <1450131285-30182-1-git-send-email-steffan@karger.me>
URL: http://article.gmane.org/gmane.network.openvpn.devel/10802
Signed-off-by: Gert Doering <gert@greenie.muc.de>