build: reduce CMake handling for zlib

Authored by compnerd on Jan 1 2020, 3:01 PM.


build: reduce CMake handling for zlib

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.