Set c_stdlib_version for macos

[isuruf]

This needs to match the global pinning

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[conda-forge-webservices]

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

I was trying to look for recipes to lint for you, but couldn't find any.
Please ping the 'conda-forge/core' team (using the @ notation in a comment) if you believe this is a bug.

[isuruf]

cc @conda-forge/staged-recipes

[github-actions]

To help direct your pull request to the best reviewers, please mention a topic-specifc team if your recipe matches any of the following: conda-forge/help-c-cpp, conda-forge/help-cdts, conda-forge/help-go, conda-forge/help-java, conda-forge/help-julia, conda-forge/help-nodejs, conda-forge/help-perl, conda-forge/help-python, conda-forge/help-python-c, conda-forge/help-r, conda-forge/help-ruby,or conda-forge/help-rust. Thanks!