GN build improvemenst

tools/gn/BUILD.gn

Index: tools/gn/BUILD.gn
diff --git a/tools/gn/BUILD.gn b/tools/gn/BUILD.gn
index e3724ad9425da6aa4f75f8e138f653ce5fbad05e..1e9391fb3e802d56490229986b48bec0af51e7a4 100644
--- a/tools/gn/BUILD.gn
+++ b/tools/gn/BUILD.gn
@@ -174,6 +174,22 @@ static_library("gn_lib") {
   ]
 }
 
+action("last_commit_position") {
+  script = "last_commit_position.py"
+
+  # This dependency forces a re-run when the code is synced.
+  inputs = [ "//build/util/LASTCHANGE" ]
+
+  outfile = "$target_gen_dir/last_commit_position.h"
+  outputs = [ outfile ]
+
+  args = [
+    rebase_path("//", root_build_dir),
+    rebase_path(outfile, root_build_dir),
+    "TOOLS_GN_LAST_COMMIT_POSITION_H_",
+  ]
+}
+
 executable("gn") {
   sources = [
     "gn_main.cc",
@@ -181,7 +197,7 @@ executable("gn") {
 
   deps = [
     ":gn_lib",
-    "//build/util:last_change",
+    ":last_commit_position",
   ]
 }