Support more configurability in GN toolchains

tools/gn/ninja_utils.h

+// 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.
+
+#ifndef TOOLS_GN_NINJA_UTILS_H_
+#define TOOLS_GN_NINJA_UTILS_H_
+
+#include <string>
+
+class Settings;
+class SourceFile;
+class Target;
+
+// Example: "base/base.ninja". The string version will not be escaped, and
+// will always have slashes for path separators.
+SourceFile GetNinjaFileForTarget(const Target* target);
+
+// Returns the name of the root .ninja file for the given toolchain.
+SourceFile GetNinjaFileForToolchain(const Settings* settings);
+
+// Returns the prefix applied to the Ninja rules in a given toolchain so they
+// don't collide with rules from other toolchains.
+std::string GetNinjaRulePrefixForToolchain(const Settings* settings);
+
+#endif  // TOOLS_GN_NINJA_UTILS_H_