Stack overflow in b2

As mentioned in https://github.com/boostorg/bcp/pull/5, I'm getting a stack overflow (due to apparently endless recursion) in b2. Reproducing it is a bit convoluted at this point:

1. You need to switch `tools/bcp` to the `feature/update-specials` branch
2. Build bcp with `b2 tools/bcp` from the root
3. Issue `dist/bin/bcp program_options build /some/dir`
4. `cd /some/dir`
5. `./bootstrap`
6. `./b2 libs/program_options/test`