Require libcxx without modified header guards

[saraedum]

This is a workaround for conda-forge/cppyy-feedstock#78.

We also need to make this same change for all the previous builds for macOS.

[conda-forge-webservices]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.

[saraedum]

@conda-forge-admin, please rerender

[saraedum]

This will probably then break again when xcode 16 is released with its llvm 17.
I think there is not much we can do about this. Basically, we need the user to install a libcxx that is compatible with their xcode. I don't think we can patch such a requirement into the existing builds of cppyy-clang somehow.
Maybe better than breaking this for future users of old versions of cppyy is to just document this in an issue here.