fixes check for what type future_status returns

When I try the current master branch on my Mac I get:

```
In file included from /Users/william/devel/ros_dds/prototype/src/rclcpp_examples/src/add_two_ints_client.cpp:1:
In file included from /Users/william/devel/ros_dds/prototype/src/rclcpp/include/rclcpp/rclcpp.hpp:8:
In file included from /Users/william/devel/ros_dds/prototype/src/rclcpp/include/rclcpp/node/node.hpp:19:
/Users/william/devel/ros_dds/prototype/src/rclcpp/include/rclcpp/client/client.hpp:66:28: error: assigning to 'bool' from incompatible type 'std::__1::future_status'
                    status = f.wait_for(std::chrono::milliseconds(100));
                           ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/Users/william/devel/ros_dds/prototype/src/rclcpp_examples/src/add_two_ints_client.cpp:28:29: note: in instantiation of member function
      'rclcpp::client::Client<rclcpp_examples::AddTwoInts>::call' requested here
    auto response = client->call(req);
                            ^
1 error generated.
```

This pull request fixes that by changing the `#ifdef` block to consider `__clang__`.