Enable the -Wundef warning for clang

webrtc/base/basictypes.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 13 matching lines @@
 #if defined(__arm__) || defined(_M_ARM)
 #define CPU_ARM 1
 #endif
 
 #if defined(CPU_X86) && defined(CPU_ARM)
 #error CPU_X86 and CPU_ARM both defined.
 #endif
 
 #if !defined(RTC_ARCH_CPU_BIG_ENDIAN) && !defined(RTC_ARCH_CPU_LITTLE_ENDIAN)
 // x86, arm or GCC provided __BYTE_ORDER__ macros
-#if CPU_X86 || CPU_ARM ||  \
+#if defined(CPU_X86) || defined(CPU_ARM) ||                             \
   (defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__)
 #define RTC_ARCH_CPU_LITTLE_ENDIAN
 #elif defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
 #define RTC_ARCH_CPU_BIG_ENDIAN
 #else
 #error RTC_ARCH_CPU_BIG_ENDIAN or RTC_ARCH_CPU_LITTLE_ENDIAN should be defined.
 #endif
 #endif
 
 #if defined(RTC_ARCH_CPU_BIG_ENDIAN) && defined(RTC_ARCH_CPU_LITTLE_ENDIAN)
@@ skipping 16 matching lines @@
 #define RTC_IS_ALIGNED(p, a) (!((uintptr_t)(p) & ((a) - 1)))
 
 // Use these to declare and define a static local variable that gets leaked so
 // that its destructors are not called at exit.
 #define RTC_DEFINE_STATIC_LOCAL(type, name, arguments) \
   static type& name = *new type arguments
 
 #endif  // __cplusplus
 
 #endif  // WEBRTC_BASE_BASICTYPES_H_