Improve interoperatability between Make and Rake

To make it easier to call Rake targets from the Makefile we should avoid constructions like these:

.PHONY: Procfile
Procfile:
	$(Q)rake $@

I think it would be easy to automate this:

1. Add a rake target that for_each Rake target generates these lines into something like tmp/Makefile.rake.mk
2. include that file into our Makefile
3. Have that file rebuilt on basically any change in lib