Require 'cff' build constraint

[abhinav]

The 'cff' build constraint is currently optional.
If someone fails to add the constraint, we won't generate the inverse of the constraint in the generated file.
This is a problem because if someone does that, they'll start seeing "duplicate declaration" errors.

We should make the 'cff' build constraint required for files processed by cff.
However, for our internal consumption of the tool, the constraint is optional.
To that end, the "constraint is required" condition should be modifiable with a simple patch.
Perhaps a global _requireBuildConstraint bool in the main package.