[llvm] [CMake] Fix config when static zstd libraries are not found (PR #113584)

[Aaron Puchert via llvm-commits]

https://github.com/aaronpuchert approved this pull request.

Looks good to me. I share @mgorny's concerns about complexity, but I don't see an easy way out.

https://github.com/llvm/llvm-project/pull/113584