Have OLDTOY emit (redundant) function prototype so single.sh can build OLDTOY standalone (if it has its own config symbol).
diff --git a/toys.h b/toys.h
index f431c74..2e5c6b0 100644
--- a/toys.h
+++ b/toys.h
@@ -75,7 +75,7 @@
// Get list of function prototypes for all enabled command_main() functions.
#define NEWTOY(name, opts, flags) void name##_main(void);
-#define OLDTOY(name, oldname, opts, flags)
+#define OLDTOY(name, oldname, opts, flags) void oldname##_main(void);
#include "generated/newtoys.h"
#include "generated/oldtoys.h"
#include "generated/flags.h"