Enable the -Wundef warning for clang

webrtc/base/criticalsection.h

 /*
  *  Copyright 2004 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.
  */
 
@@ skipping 21 matching lines @@
 
 // See notes in the 'Performance' unit test for the effects of this flag.
 #define USE_NATIVE_MUTEX_ON_MAC 0
 
 #if defined(WEBRTC_MAC) && !USE_NATIVE_MUTEX_ON_MAC
 #include <dispatch/dispatch.h>
 #endif
 
 #if (!defined(NDEBUG) || defined(DCHECK_ALWAYS_ON))
 #define CS_DEBUG_CHECKS 1
+#else
+#define CS_DEBUG_CHECKS 0
 #endif
 
 #if CS_DEBUG_CHECKS
 #define CS_DEBUG_CODE(x) x
 #else  // !CS_DEBUG_CHECKS
 #define CS_DEBUG_CODE(x)
 #endif  // !CS_DEBUG_CHECKS
 
 namespace rtc {
 
@@ skipping 91 matching lines @@
   explicit GlobalLockScope(GlobalLockPod* lock) EXCLUSIVE_LOCK_FUNCTION(lock);
   ~GlobalLockScope() UNLOCK_FUNCTION();
  private:
   GlobalLockPod* const lock_;
   RTC_DISALLOW_COPY_AND_ASSIGN(GlobalLockScope);
 };
 
 } // namespace rtc
 
 #endif // WEBRTC_BASE_CRITICALSECTION_H_