add cxxstd variant to camp

[dtaller]

The most recent version of camp has concepts, a C++20 feature, so I added a variant to specify the version so I can build with spack.

[dtaller]

@spackbot fix style

[spackbot-app]

Let me see if I can fix that for you!

[spackbot-app]

I was able to run spack style --fix for you!

spack style --fix

repos/spack_repo/builtin/packages/camp/package.py
�[1;34m==> �[0mrunning ruff format
1 file reformatted
�[1;34m==> �[0mrunning ruff check
�[1m�[91mE501 �[0m�[1mLine too long (126 > 99)�[0m
  �[1m�[94m-->�[0m repos/spack_repo/builtin/packages/camp/package.py:97:100
   �[1m�[94m|�[0m
�[1m�[94m95 |�[0m     variant("sycl", default=False, description="Build with Sycl support")
�[1m�[94m96 |�[0m
�[1m�[94m97 |�[0m     # TODO: Add conflicts. I know that  v2026.07.0 has concepts (C++20 feature) and cannot be built with earlier C++ versions.
   �[1m�[94m|�[0m                                                                                                    �[1m�[91m^^^^^^^^^^^^^^^^^^^^^^^^^^^�[0m
�[1m�[94m98 |�[0m     variant(
�[1m�[94m99 |�[0m         "cxxstd",
   �[1m�[94m|�[0m

Found 1 error.

Keep in mind that I cannot fix your flake8 or mypy errors, so if you have any you'll need to fix them and update the pull request. If I was able to push to your branch, if you make further changes you will need to pull from your updated branch before pushing again.

I've updated the branch with style fixes.

[dtaller]

Converting to draft. I think the cxxstd fix may need to go into packages that use camp's header files, such as axom