Disable building INDEX64_EXT_API for better compatibility

[mgorny]

Checklist

• Used a personal fork of the feedstock to propose changes
• Bumped the build number (if the version is unchanged)
• Reset the build number to 0 (if the version changed)
• Re-rendered with the latest conda-smithy (Use the phrase @conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• Ensured the license file is being packaged.

Disable building INDEX64_EXT_API, as it includes ILP64 symbols within the LP64 library. We cannot do that unless all the alternative providers do that as well, as otherwise packages can start linking to these symbols. We already hit this in the wild while trying to test BLIS with LAPACK 3.12.1. See:
conda-forge/blas-feedstock#152 (comment)

[conda-forge-admin]

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/meta.yaml) and found it was in an excellent condition.

I do have some suggestions for making it better though...

For recipe/meta.yaml:

• ℹ️ The recipe is not parsable by parser conda-recipe-manager. The recipe can only be automatically migrated to the new v1 format if it is parseable by conda-recipe-manager.

_{This message was generated by GitHub Actions workflow run https://github.com/conda-forge/conda-forge-webservices/actions/runs/19940883589. Examine the logs at this URL for more detail.}

[h-vetinari]

Thanks