Revert "Merge pull request #16507 from vrthra/unifdef"

This reverts commit cab28503, reversing
changes made to 5313f109 (but retaining
the meta updates).

The update to 2.11 breaks the darwin toolchain, see discussion at
https://github.com/NixOS/nixpkgs/pull/16507

Once the issues with 2.11 are worked out, this revert can be reverted.