updated CMake to set thread safety on when meshgen is on

Issue/feature addressed

Boost thread safety has to be enabled for the variational optimisation module in NekMesh to work, otherwise it causes a segmentation fault when executed in parallel. Currently, this has to be enabled manually and requires recompiling in case one forgets.

Proposed solution

Enable boost thread safety whenever MeshGen is enabled as the variational framework module (one of the central components of NekMesh) cannot be used without it.

Implementation

CMake changed to set NEKTAR_USE_THREAD_SAFETY to ON when MeshGen is set to ON.

Tests

NA

Suggested reviewers

Anyone! This is very simple.

Notes

NA

Checklist

• Functions and classes, or changes to them, are documented.
• User guide/documentation is updated.
• Changelog is updated.
• Suitable tests added for new functionality.
• Contributed code is correctly formatted. (See the contributing guidelines).
• License added to any new files.
• No extraneous files have been added (e.g. compiler output or test data files).