Makefile: suppress warning for undo-redo being undefined

It is part of the newer Emacs, but on the older ones it causes
warnings. There's no tests of that functionality, so just suppress the
warning.

Makefile

@@ -26,7 +26,7 @@ compile: $(ELCFILES)
 -include .depend
 
 $(ELCFILES): %.elc: %.el
-	$(EMACS) --batch -Q -L . -f batch-byte-compile $<
+	$(EMACS) --batch -Q -L . --eval "(declare-function undo-redo nil)" -f batch-byte-compile $<
 
 # Byte-compile all files in one batch. This is faster than
 # compiling each file in isolation, but also less stringent.