Enable the -Wundef warning for clang

webrtc/base/BUILD.gn

 # 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.
 
 import("//build/config/crypto.gni")
 import("//build/config/ui.gni")
@@ skipping 99 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.cc",
     "location.h",
     "md5.cc",
     "md5.h",
     "md5digest.cc",
     "md5digest.h",
     "mod_ops.h",
     "onetimeevent.h",
     "optional.cc",
     "optional.h",
@@ skipping 609 matching lines @@
       "java/src/org/webrtc/Logging.java",
       "java/src/org/webrtc/Size.java",
       "java/src/org/webrtc/ThreadUtils.java",
     ]
 
     deps = [
       "//base:base_java",
     ]
   }
 }