Fix #210: Unable to add_subdirectory (cucumber-cpp)

[ghost]

This fixes issue #210

Types of changes

• Bug fix (non-breaking change which fixes an issue).
• New feature (non-breaking change which adds functionality).
• Breaking change (fix or feature that would cause existing functionality to not work as expected).

Checklist:

• It is my own work, its copyright is implicitly assigned to the project and no substantial part of it has been copied from other sources (including Stack Overflow). In rare occasions this is acceptable, like in CMake modules where the original copyright information should be kept.
• I'm using the same code standards as the existing code (indentation, spacing, variable naming, ...).
• I've added tests for my code.
• I have verified whether my change requires changes to the documentation
• My change either requires no documentation change or I've updated the documentation accordingly.
• My branch has been rebased to master, keeping only relevant commits.

[coveralls]

Coverage remained the same at 62.814% when pulling c3a3e21 on sergey-bon:fix-sub-directory into f799908 on cucumber:master.

[aslakhellesoy]

Hi @sergey-bon,

Thanks for your making your first contribution to Cucumber, and welcome to the Cucumber committers team! You can now push directly to this repo and all other repos under the cucumber organization! 🍾

In return for this generous offer we hope you will:

• ✅ Continue to use branches and pull requests. When someone on the core team approves a pull request (yours or someone else's), you're welcome to merge it yourself.
• 💚 Commit to setting a good example by following and upholding our code of conduct in your interactions with other collaborators and users.
• 💬 Join the community Slack channel to meet the rest of the team and make yourself at home.
• ℹ️ Don't feel obliged to help, just do what you can if you have the time and the energy.
• 🙋 Ask if you need anything. We're looking for feedback about how to make the project more welcoming, so please tell us!

On behalf of the Cucumber core team,
Aslak Hellesøy
Creator of Cucumber