add-on version mismatches

[Roland McGrath]

> I have thought about this but couldn't find an easy way which isn't
> ending in making the release process difficult and/or error prone.  If
> it's not 100% safe that I can never forget making the right changes
> before a release things only get worse.

What's wrong with having the consistency check done early in "make dist" so
you can't even try to make a bad dist?  And for the add-ons you use
(linuxthreads), you won't have managed to configure your build until you
got it right.