Include the value of `ZLIB_ROOT` in `LLVMConfig.cmake` so `FindZLIB` can pick it up. This fixes an issue where ZLIB is not found on AIX runtimes despite specifying `-DZLIB_ROOT`. Reviewed By: daltenty Differential Revision: https://reviews.llvm.org/D116235
See docs/CMake.html for instructions on how to build LLVM with CMake.