Stack overflow in b2

[pdimov]

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

[stale[bot]]

Thank you for your contributions. Main development of B2 has moved to https://github.com/bfgroup/b2 This issue has been automatically marked as "transition" to indicate the potential for needing transition to the new B2 development project.