Support more configurability in GN toolchains

tools/gn/ninja_utils.cc

+// Copyright 2014 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.
+
+#include "tools/gn/ninja_utils.h"
+
+#include "tools/gn/filesystem_utils.h"
+#include "tools/gn/settings.h"
+#include "tools/gn/target.h"
+
+SourceFile GetNinjaFileForTarget(const Target* target) {
+  return SourceFile(GetTargetOutputDir(target).value() +
+                    target->label().name() + ".ninja");
+}
+
+SourceFile GetNinjaFileForToolchain(const Settings* settings) {
+  return SourceFile(GetToolchainOutputDir(settings).value() +
+                    "toolchain.ninja");
+}
+
+std::string GetNinjaRulePrefixForToolchain(const Settings* settings) {
+  // Don't prefix the default toolchain so it looks prettier, prefix everything
+  // else.
+  if (settings->is_default())
+    return std::string();  // Default toolchain has no prefix.
+  return settings->toolchain_label().name() + "_";
+}