Suggest check newness of "missing" etc

[Kevin Ryde]

I think it'd be good if automake checked its support files like
"missing" and "ansi2knr.c" are up-to-date, not just that they exist.
A warning if they differ from the /usr/share/automake versions would
probably be enough, since there's at least a chance of intentional
local modifications.  I guess "automake -f" is the right way to update
as such, but warnings would be a useful reminder.