Use a stamp file when symlinking in GN work around mtime issues

Ninja follows a symlink when stat'ing, so you can't use a symlink as an
output and have dirty checking work properly.
ninja-build/ninja#1186

BUG=646165

Review-Url: https://codereview.chromium.org/2348793002
Cr-Commit-Position: refs/heads/master@{#419156}

Author: agrieve

build/symlink.gni

@@ -14,11 +14,17 @@ template("symlink") {
                              "testonly",
                              "visibility",
                            ])
+
+    # Can't use the symlink itself as an output since ninja does not properly
+    # process the symlink's mtime.
+    # https://github.com/ninja-build/ninja/issues/1186
     outputs = [
-      invoker.output,
+      "$target_gen_dir/$target_name.stamp",
     ]
     script = "//build/symlink.py"
     args = [
+      "--touch",
+      rebase_path(outputs[0], root_build_dir),
       "-f",
       rebase_path(invoker.source, get_path_info(invoker.output, "dir")),
       rebase_path(invoker.output, root_build_dir),