Use correct ifdefs for LibreSSL support

Use correct ifdefs for LibreSSL support

- TLS 1.3 is not ready yet in LibreSSL. Also there is a theoretical
possibility of OpenSSL >=1.1.1 built without TLS 1.3 support.
- EC_KEY_METHOD API and SSL_get1_supported_ciphers are added into LibreSSL
master (not yet released in 2.9.0).
- Some methods that are available since LibreSSL 2.7.0 were thrown away
in ssl_openssl.c regardless of LibreSSL version. Use them with newer
LibreSSL.

Signed-off-by: Stefan Strogin <stefan.strogin@gmail.com>
Acked-by: Arne Schwabe <arne@rfc2549.org>
Message-Id: <20190225183531.27399-1-stefan.strogin@gmail.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg18239.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>