Add rtc_enable_external_auth variable for setting ENABLE_EXTERNAL_AUTH

webrtc/BUILD.gn

Index: webrtc/BUILD.gn
diff --git a/webrtc/BUILD.gn b/webrtc/BUILD.gn
index 3fb9cfea72ebf94abda5ea30f50f0f079ca32b94..ba58565dc8b47b8fa970258c4a009027e1fbcddd 100644
--- a/webrtc/BUILD.gn
+++ b/webrtc/BUILD.gn
@@ -122,13 +122,16 @@ config("common_config") {
     defines += [ "HAVE_SCTP" ]
   }
 
+  if (rtc_enable_external_auth) {
+    defines += [ "ENABLE_EXTERNAL_AUTH" ]
+  }
+
   if (build_with_chromium) {
     defines += [
       # NOTICE: Since common_inherited_config is used in public_configs for our
       # targets, there's no point including the defines in that config here.
       # TODO(kjellander): Cleanup unused ones and move defines closer to the
       # source when webrtc:4256 is completed.
-      "ENABLE_EXTERNAL_AUTH",
       "HAVE_OPENSSL_SSL_H",
       "HAVE_SRTP",
       "HAVE_WEBRTC_VIDEO",