... and make sure to recreate res if needed

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
diff --git a/Makefile b/Makefile
index 3606514..61837fb 100644
--- a/Makefile
+++ b/Makefile
@@ -4,5 +4,8 @@
 clean:
 	@rm -f res/*
 
-res/%: %.m4 defs calls
+res:
+	@mkdir res
+
+res/%: %.m4 defs calls res
 	@m4 $< >$@