zlib and libpng don't compile with bleeding edge clang on macOS

This change in clang: https://github.com/llvm/llvm-project/pull/74676

Breaks the build of VTK master on macOS.

We need these fixes:

zilb: https://github.com/madler/zlib/pull/895

libpng: https://github.com/pnggroup/libpng/pull/529

Would be nice to get ahead of this before this clang change makes in into Xcode.