gn: Support build directories outside the source tree.

tools/gn/value_extractors.h

Index: tools/gn/value_extractors.h
diff --git a/tools/gn/value_extractors.h b/tools/gn/value_extractors.h
index 2482ba25bf2b3a82e19d6adc29585eb5fe6f67ed..affc1e34d0c3530c8fb43be9b36892b3f0670b90 100644
--- a/tools/gn/value_extractors.h
+++ b/tools/gn/value_extractors.h
@@ -39,7 +39,8 @@ bool ExtractListOfRelativeDirs(const BuildSettings* build_settings,
 
 // Extracts the list of labels and their origins to the given vector. Only the
 // labels are filled in, the ptr for each pair in the vector will be null.
-bool ExtractListOfLabels(const Value& value,
+bool ExtractListOfLabels(const BuildSettings* build_settings,
+                         const Value& value,
                          const SourceDir& current_dir,
                          const Label& current_toolchain,
                          LabelTargetVector* dest,
@@ -49,17 +50,20 @@ bool ExtractListOfLabels(const Value& value,
 // version taking Label*Pair, only the labels are filled in, the ptr for each
 // pair in the vector will be null. Sets an error and returns false if a label
 // is maformed or there are duplicates.
-bool ExtractListOfUniqueLabels(const Value& value,
+bool ExtractListOfUniqueLabels(const BuildSettings* build_settings,
+                               const Value& value,
                                const SourceDir& current_dir,
                                const Label& current_toolchain,
                                UniqueVector<Label>* dest,
                                Err* err);
-bool ExtractListOfUniqueLabels(const Value& value,
+bool ExtractListOfUniqueLabels(const BuildSettings* build_settings,
+                               const Value& value,
                                const SourceDir& current_dir,
                                const Label& current_toolchain,
                                UniqueVector<LabelConfigPair>* dest,
                                Err* err);
-bool ExtractListOfUniqueLabels(const Value& value,
+bool ExtractListOfUniqueLabels(const BuildSettings* build_settings,
+                               const Value& value,
                                const SourceDir& current_dir,
                                const Label& current_toolchain,
                                UniqueVector<LabelTargetPair>* dest,