FIX: Explicit bfs:path conversion to string.

Summary

Fixed the compiler and also a different linker issue.

Test Environment

Simply try to compile on a Mac with make conan.

Check List for the Author

• All automated tests pass
• Reference related Issues: #25 (closed)
• Up-to-date CHANGELOG.md
• Annotations in code (Gitlab comments)
  • Intent of new code
  • Problems with old code
  • Why this implementation?

Check List for the Reviewer

• I understand the intent of this MR
• All automated tests pass
• Up-to-date CHANGELOG.md
• The test environment setup works and the intended behavior is reproducible in the test environment
• In-code documentation and comments are up-to-date.
• Check: Are there specifications? Are they satisfied?

For further good practices have a look at our review guidelines.