Enable the -Wundef warning for clang

webrtc/base/base.gyp

 # Copyright (c) 2014 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.
 
 {
   'includes': [ '../build/common.gypi', ],
@@ skipping 41 matching lines @@
         'event.cc',
         'event.h',
         'event_tracer.cc',
         'event_tracer.h',
         'exp_filter.cc',
         'exp_filter.h',
         'file.cc',
         'file.h',
         'format_macros.h',
         'function_view.h',
+        'ignore_wundef.h',
         'location.h',
         'location.cc',
         'md5.cc',
         'md5.h',
         'md5digest.cc',
         'md5digest.h',
         'mod_ops.h',
         'onetimeevent.h',
         'optional.cc',
         'optional.h',
@@ skipping 594 matching lines @@
     },
     {
      'target_name': 'gtest_prod',
       'type': 'static_library',
       'sources': [
         'gtest_prod_util.h',
       ],
     },
   ],
 }