Rather than handling zlib handling manually, use `find_package` from CMake
to find zlib properly. Use this to normalize the `LLVM_ENABLE_ZLIB`,
`HAVE_ZLIB`, `HAVE_ZLIB_H`. Furthermore, require zlib if `LLVM_ENABLE_ZLIB` is
set to `YES`, which requires the distributor to explicitly select whether
zlib is enabled or not. This simplifies the CMake handling and usage in
the rest of the tooling.
This restores 68a235d07f9e7049c7eb0c8091f37e385327ac28,
e6c7ed6d2164a0659fd9f6ee44f1375d301e3cad. The problem with the windows
bot is a need for clearing the cache.
ead085e3a4