Add note to 1984/anonymous/Makefile about macOS

There are a number of issues that cause this. It might work with a more
recent compiler but it might not - I haven't tested that but anyone with
the default install will probably have problems.

1984/anonymous/Makefile

@@ -443,6 +443,7 @@ all: ${DATA} ${TARGET}
 	supernova deep_magic magic charon pluto
 
 ${PROG}: ${PROG}.c
+	@echo "NOTE: This entry most likely will not work under macOS."
 	${CC} ${CFLAGS} $< -o $@ ${LIBS}
 
 # alternative executable