Enable the -Wundef warning for clang

webrtc/common_types.h

 /*
  *  Copyright (c) 2012 The WebRTC project authors. All Rights Reserved.
  *
  *  Use of this source code is governed by a BSD-style license
  *  that can be found in the LICENSE file in the root of the source
  *  tree. An additional intellectual property rights grant can be found
  *  in the file PATENTS.  All contributing project authors may
  *  be found in the AUTHORS file in the root of the source tree.
  */
 
 #ifndef WEBRTC_COMMON_TYPES_H_
 #define WEBRTC_COMMON_TYPES_H_
 
 #include <assert.h>
 #include <stddef.h>
 #include <string.h>
 
 #include <string>
 #include <vector>
 
 #include "webrtc/common_video/rotation.h"
 #include "webrtc/typedefs.h"
 
 #if defined(_MSC_VER)
 // Disable "new behavior: elements of array will be default initialized"
 // warning. Affects OverUseDetectorOptions.
 #pragma warning(disable : 4351)
 #endif
 
-#ifdef WEBRTC_EXPORT
+#if defined(WEBRTC_EXPORT)
 #define WEBRTC_DLLEXPORT _declspec(dllexport)
-#elif WEBRTC_DLL
+#elif defined(WEBRTC_DLL)
 #define WEBRTC_DLLEXPORT _declspec(dllimport)
 #else
 #define WEBRTC_DLLEXPORT
 #endif
 
 #ifndef NULL
 #define NULL 0
 #endif
 
 #define RTP_PAYLOAD_NAME_SIZE 32u
@@ skipping 778 matching lines @@
 enum class RtcpMode { kOff, kCompound, kReducedSize };
 
 enum NetworkState {
   kNetworkUp,
   kNetworkDown,
 };
 
 }  // namespace webrtc
 
 #endif  // WEBRTC_COMMON_TYPES_H_