Support more path-specific flags

Once #313 is merged, pre-processing and linking does not yet support path-specific flags. This would be useful (though not really required) for pfunit-based tests (which need a preprocessor define for the main program, and the pfuint library to be linked in).

'not really required' meaning it doesn't hurt if additional preprocessor directives are used everywhere, or a library that is not used is linked in.

Start by reading issue #313 and then trace the preprocessing and linking entry points in fab. Determine how flags can be scoped to individual paths, using the pfunit example as the expected behavior; done means path-specific preprocessor defines and libraries are supported without requiring them globally.

Written by the indexing model from the issue text.