solved I think: rebuild if any variable changes. Is it right?

[Britton Kerin]

The rebuild-the-world-if-anything-changed approach turned out to be
pretty simple I think.  Opinions welcome on the below solution, I
haven't used it much yet.  It can be tricked by pathological stuff
like ifdef SOME_VAR ... endif SOME_VAR=constant_val (without any
effects on other variables in ...) earlier in the Makefile but is it
otherwise ok?

# For putting newlines in strings:
define nl


endef

# Roots of the DAG should all depend on smv (Serialized Make Variables)
bar: foo smv
# Fake build for demo:
        cp $< $@

clean:
        rm -f smv

# This should be at the end of the Makefile (or after all var declarations):
$(file                                                                       \
  >nsmv,                                                                     \
  $(foreach v,$(.VARIABLES),$(flavor $(v)) var $(v) has value $($(v))$(nl)))
# Note: not removing nsmv would be tiny bit more mess but might be nice to
# be able to see what's going on.
$(shell (([ -a smv ] && (diff smv nsmv >/dev/null)) || cp nsmv smv); rm nsmv)