In the presence of a new CLI flag, the builder will now override certain
operations so that they are driven by fontc.

This does two things:

- it takes a path, where all our final build products will be saved
- it reduces the set of build targets to match what we would build
  with fontc

this requires an ugly global in order to expose this to the relevant
operations, but that was the best solution I could come up with.

This instructs gftools to only build the single named source file.

This way if we are running multiple instances of gftools agains the same
sources, we aren't overwriting each other's build files.