SSL: fixed SSL_clear_options() usage with OpenSSL 1.1.0+.

In OpenSSL 1.1.0 the SSL_CTRL_CLEAR_OPTIONS macro was removed, so
conditional compilation test on it results in SSL_clear_options()
and SSL_CTX_clear_options() not being used.  Notably, this caused
"ssl_prefer_server_ciphers off" to not work in SNI-based virtual
servers if server preference was switched on in the default server.

It looks like the only possible fix is to test OPENSSL_VERSION_NUMBER
explicitly.