Enable the -Wundef warning for clang

webrtc/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.
 
 # TODO(kjellander): Rebase this to webrtc/build/common.gypi changes after r6330.
 
@@ skipping 161 matching lines @@
           "-Woverloaded-virtual",
         ]
       }
     }
 
     if (is_clang) {
       cflags += [
         "-Wimplicit-fallthrough",
         "-Wthread-safety",
         "-Winconsistent-missing-override",
+        "-Wundef",
       ]
     }
   }
 
   if (current_cpu == "arm64") {
     defines += [ "WEBRTC_ARCH_ARM64" ]
     defines += [ "WEBRTC_HAS_NEON" ]
   }
 
   if (current_cpu == "arm") {
@@ skipping 628 matching lines @@
       ]
 
       deps = [
         "//webrtc/api:libjingle_peerconnection_java",
         "//webrtc/api:libjingle_peerconnection_jni",
         "//webrtc/examples:AppRTCMobile_javalib",
       ]
     }
   }
 }