Add an external_startup_data config to the GN buildfile.

BUILD.gn

Index: BUILD.gn
diff --git a/BUILD.gn b/BUILD.gn
index f5e6f5c01b9eb0dd4082b7d39f1a30cefbd72b33..34000b3fe3f8cf9ff958f3344da2fa7151a4b28b 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -71,6 +71,14 @@ config("external_config") {
   include_dirs = [ "include" ]
 }
 
+# This config should only be applied to code that needs to be explicitly
+# aware of whether we are using startup data or not.
+config("external_startup_data") {
+  if (v8_use_external_startup_data) {
+    defines = [ "V8_USE_EXTERNAL_STARTUP_DATA" ]
+  }
+}
+
 config("features") {
   visibility = [ ":*" ]  # Only targets in this file can depend on this.