SSL: safeguard use of SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS.

The flag was recently removed by BoringSSL.
diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index 20f9eea..d5d4a1a 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -1146,11 +1146,15 @@
         c->recv_chain = ngx_ssl_recv_chain;
         c->send_chain = ngx_ssl_send_chain;
 
+#ifdef SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS
+
         /* initial handshake done, disable renegotiation (CVE-2009-3555) */
         if (c->ssl->connection->s3) {
             c->ssl->connection->s3->flags |= SSL3_FLAGS_NO_RENEGOTIATE_CIPHERS;
         }
 
+#endif
+
         return NGX_OK;
     }