GN: Add ability to specify a depfile for custom targets.

tools/gn/script_target_generator.cc

 // Copyright (c) 2013 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/script_target_generator.h"
 
+#include "tools/gn/build_settings.h"
 #include "tools/gn/err.h"
 #include "tools/gn/filesystem_utils.h"
 #include "tools/gn/scope.h"
 #include "tools/gn/value.h"
 #include "tools/gn/value_extractors.h"
 #include "tools/gn/variables.h"
 
 ScriptTargetGenerator::ScriptTargetGenerator(Target* target,
                                              Scope* scope,
                                              const Token& function_token,
@@ skipping 24 matching lines @@
     return;
 
   FillScriptArgs();
   if (err_->has_error())
     return;
 
   FillOutputs();
   if (err_->has_error())
     return;
 
+  FillDepfile();
+  if (err_->has_error())
+    return;
+
   // Script outputs don't depend on the current toolchain so we can skip adding
   // that dependency.
 }
 
 void ScriptTargetGenerator::FillScript() {
   // If this gets called, the target type requires a script, so error out
   // if it doesn't have one.
   const Value* value = scope_->GetValue(variables::kScript, true);
   if (!value) {
     *err_ = Err(function_token_, "This target type requires a \"script\".");
     return;
   }
   if (!value->VerifyTypeIs(Value::STRING, err_))
     return;
 
   target_->script_values().set_script(
       scope_->GetSourceDir().ResolveRelativeFile(value->string_value()));
 }
 
 void ScriptTargetGenerator::FillScriptArgs() {
   const Value* value = scope_->GetValue(variables::kArgs, true);
   if (!value)
     return;
 
   std::vector<std::string> args;
   if (!ExtractListOfStringValues(*value, &args, err_))
     return;
   target_->script_values().swap_in_args(&args);
 }
+
+void ScriptTargetGenerator::FillDepfile() {
+  const Value* value = scope_->GetValue(variables::kDepfile, true);
+  if (!value)
+    return;
+  target_->script_values().set_depfile(
+      scope_->settings()->build_settings()->build_dir().ResolveRelativeFile(
+          value->string_value()));
+}