gn: Support build directories outside the source tree.

tools/gn/function_get_target_outputs.cc

Index: tools/gn/function_get_target_outputs.cc
diff --git a/tools/gn/function_get_target_outputs.cc b/tools/gn/function_get_target_outputs.cc
index 163b187d82049940c772f5b7db868d91c656ca42..40ce9f2b9498eac87e041b20e572747fb00d0196 100644
--- a/tools/gn/function_get_target_outputs.cc
+++ b/tools/gn/function_get_target_outputs.cc
@@ -79,7 +79,9 @@ Value RunGetTargetOutputs(Scope* scope,
 
   // Resolve the requested label.
   Label label = Label::Resolve(scope->GetSourceDir(),
-                               ToolchainLabelForScope(scope), args[0], err);
+                               ToolchainLabelForScope(scope), args[0],
+                               scope->settings()->build_settings()->root_path(),
+                               err);
   if (label.is_null())
     return Value();