[C++] Re-run commands only when certain directories are updated
diff --git a/strutil.h b/strutil.h
index 16f1f1e..2dd229c 100644
--- a/strutil.h
+++ b/strutil.h
@@ -136,4 +136,6 @@
 
 string SortWordsInString(StringPiece s);
 
+string ConcatDir(StringPiece b, StringPiece n);
+
 #endif  // STRUTIL_H_