Add a target in GN that generates a last_change.h file for the SVN revision.

tools/gn/BUILD.gn

 # Copyright (c) 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 gyp_file = "gn.gyp"
 
+# Not defined when doing a pure GYP build, this lets the code key off of
+# GN-specific features (the last change target).
+defines = [ "GN_BUILD" ]
+
 static_library("gn_lib") {
   sources = [
     "args.cc",
     "args.h",
     "binary_target_generator.cc",
     "binary_target_generator.h",
     "build_settings.cc",
     "build_settings.h",
     "command_args.cc",
     "command_desc.cc",
@@ skipping 126 matching lines @@
     "toolchain_manager.h",
     "trace.cc",
     "trace.h",
     "value.cc",
     "value.h",
     "value_extractors.cc",
     "value_extractors.h",
     "variables.cc",
     "variables.h",
   ]
+
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
+    "//build/util:last_change",
   ]
 }
 
 executable("gn") {
   sources = [
     "gn_main.cc",
   ]
+
   deps = [
     ":gn_lib",
   ]
 }
 
 test("gn_unittests") {
   sources = [
     "escape_unittest.cc",
     "file_template_unittest.cc",
     "filesystem_utils_unittest.cc",
@@ skipping 22 matching lines @@
     "//base:run_all_unittests",
     "//base:test_support_base",
     "//testing:gtest",
   ]
 }
 
 executable("generate_test_gn_data") {
   sources = [ "generate_test_gn_data.cc" ]
   deps = [ "//base" ]
 }