make delete-old safety checks

[Eitan Adler]

Hey all,

I recently shot myself in the foot by running make delete-old on a
tree that was newer than the installed version of my system. It
happened to be that this resulted in deleting many compiler headers,
thus making it impossible to recompile and recover without resorting
to backups.

Is it possible to add safety checks, similar to the recent additions
of 'installworld before buildworld', for this case?

-- 
Eitan Adler