Add clz functions (Count number of Leading Zero bits), 32-and 64-bit variants

webrtc/system_wrappers/include/compile_assert_c.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_SYSTEM_WRAPPERS_INCLUDE_COMPILE_ASSERT_H_
 #define WEBRTC_SYSTEM_WRAPPERS_INCLUDE_COMPILE_ASSERT_H_
 
-#ifdef __cplusplus
-#error "Only use this for C files. For C++, use static_assert."
-#endif
-
 // Use this macro to verify at compile time that certain restrictions are met.
 // The argument is the boolean expression to evaluate.
 // Example:
 //   COMPILE_ASSERT(sizeof(foo) < 128);
+// Note: In C++, use static_assert instead!
 #define COMPILE_ASSERT(expression) switch (0) {case 0: case expression:;}
 
 #endif  // WEBRTC_SYSTEM_WRAPPERS_INCLUDE_COMPILE_ASSERT_H_