Comment 1 for bug 502946

It's not clear that you need to recompile if the inter-system dependency
changes, unless the API has changed, and in particular in ways that
affect the behavior of downstream systems (e.g., macro definitions change).

ISTR that some Lisp defsystems made it possible for the programmer to
annotate them as providing things like macro definitions that should
trigger recompilation downstream. I think it would be good to add
something like that to ASDF. I believe that this is the semantics of
the :definitions spec in Franz's version of defsystem, but I can't say
for sure, never having used that defsystem.