clang wants "complete" declarations at class header, in particular
default ctors

Index: src/document.h
--- src/document.h.orig
+++ src/document.h
@@ -65,7 +65,7 @@ class Document : protected CommandSystem (public)
 	/*
 	 * Constructor by copy
 	 */
-	Document(Document &copy, bool copy_subtitles );
+	Document(Document &copy, bool copy_subtitles = true);
 
 	/*
 	 * Destructor