Curl_setopt: refuse CURL_HTTP_VERSION_2_0 if built without support

lib/url.c

@@ -1203,7 +1203,12 @@ CURLcode Curl_setopt(struct SessionHandle *data, CURLoption option,
      * This sets a requested HTTP version to be used. The value is one of
      * the listed enums in curl/curl.h.
      */
-    data->set.httpversion = va_arg(param, long);
+    arg = va_arg(param, long);
+#ifndef USE_NGHTTP2
+    if(arg == CURL_HTTP_VERSION_2_0)
+      return CURLE_UNSUPPORTED_PROTOCOL;
+#endif
+    data->set.httpversion = arg;
     break;
 
   case CURLOPT_HTTPAUTH: