fix: stopped _XOPEN_SOURCE from defining on OpenBSD

[Jaskowicz1]

This PR fixes #765, stopping "_XOPEN_SOURCE" from compiling on OpenBSD. This has been tested and is fully working on OpenBSD. I have tested this fix on Ubuntu 22.04 and WSL and they can still compile and run sucessfully.

Code change checklist

• I have ensured that all methods and functions are fully documented using doxygen style comments.
• My code follows the coding style guide.
• I tested that my change works before raising the PR.
• I have ensured that I did not break any existing API calls.
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight.

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