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

tools/gn/BUILD.gn

Index: tools/gn/BUILD.gn
diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index 6195ff36c0dc1b02873968440ed4040718b12c2e..c5cc1cb5971918392d8799dc2f7067620aaa1286 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -4,6 +4,10 @@
 
 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",
@@ -151,9 +155,11 @@ static_library("gn_lib") {
     "variables.cc",
     "variables.h",
   ]
+
   deps = [
     "//base",
     "//base/third_party/dynamic_annotations",
+    "//build/util:last_change",
   ]
 }
 
@@ -161,6 +167,7 @@ executable("gn") {
   sources = [
     "gn_main.cc",
   ]
+
   deps = [
     ":gn_lib",
   ]