Every makefile recipe should produce exactly one output: $@. The makefile as a whole produces an arbitrary number of outputs since rules can depend on other rules.

This leads us to a neat rule of thumb for phony targets: any recipe that does not touch $@ and only $@ should have $@ marked as phony.

I find that keeping track of phony targets with a list makes things much easier.

  phonies :=

  phonies += something
  something:
          ./do-something

  phonies += something-else
  something-else: something
          ./do-something-else

  # touches $@ and thus does not need to be phony
  create-file:
          ./generate-some-output > $@

  .PHONY: $(phonies)