Fix gmake multiple-output rule issue.

[TurkeyMan]

Rules with multiple outputs were emitting rules in the form:

a b c: src
  build src -o a b c

Which is incorrect, because it will execute the rule many times (once for each output).

I think it's intended to describe that a rule produces multiple outputs, and this patch improves the modelling of that relationship in make to read like:

a: src
  build src -o a b c
b c: a

This isn't the best implementation, but it's not just plain broken, and others are much more complicated.

I left some TODO notes for future improvement.