H2O: Inhibit some compiler optimizations (#3275)

Semantically, the code used to do exactly what the JSON serialization
test required (instantiating an object for every request), but the
optimizer could remove the creation of the object completely, and
propagate its field values (known at compile time) to the JSON
generator arguments.

frameworks/C/h2o/src/request_handler.c

@@ -47,7 +47,9 @@ static int json_serializer(struct st_h2o_handler_t *self, h2o_req_t *req)
 	                                                      req->conn->ctx);
 	json_generator_t * const gen = get_json_generator(&ctx->json_generator,
 	                                                  &ctx->json_generator_num);
-	const struct {
+	// volatile is used to ensure that the object is instantiated every time
+	// the function is called.
+	const volatile struct {
 		const char *message;
 	} object = {HELLO_RESPONSE};