Yes, a big comment in the Makefile is a very good idea. I am going to be offline for ~2 hours, but will put in a comment this afternoon. (And try to figure out the bug itself.)

Ah, thank you Syd! I do have a vague memory of that. So REGENERATING=1

just proceeds without doing diffs and failing.

Perhaps we should put a comment to that effect in the Makefile.