Makefile: provide a hook to add and extend targets externally.

[dbuenzli]

Exemplified on assemblage itself.

That's just an idea, not necessarily to be merged.

Basically we just conditionally include a Makefile called Makefile.assemble at the end of the generated file. I used it on assemblage whose bootstrapping process leaves garbage in my repo that is not cleaned by a distclean.

[dbuenzli]

The use case is also if you want to add a few convenience targets for development. Maybe it could also be useful to define my build + action needs.

[samoht]

Good idea, we'll see how it works in practice later.