update patch to take into account a limitation of autosubst

[kyoDralliam]

I forgot to include an update of the patch file in the id pr (#41) that's needed because of what seems to be a limitation of autosubst (see there).

[MevenBertrand]

Since we do not use all this free variable stuff, would it be simply cleaner to stop using that option until the bug is fixed in AutoSubst? (Hopefully, the need for that ugly patch will disappear soon enough anyway)

[MevenBertrand]

Also, unrelated, but I forgot to fix it before, and now is probably a good time to do so: should we rename the AutoSubst syntax file from PiUniv.sig to something more relevant?

[kyoDralliam]

If we do not use any of the facilities provided by -allfv, indeed we could drop the option to regenerate the autosubt files. For now I would rather keep this PR (and maybe remove the allfv business in another PR) in order to keep this solution in the git history.
I don't have any strong opinion on the name of the signaure file.