zlib: HAVE_UNISTD_H is required for successful build on Apple Silicon

^^

Otherwise, some functions will be undefined and zlib devs insist that there are no errors when building with `./configure` (https://github.com/madler/zlib/pull/509). So from this point of view we have to pass the right flags if we don't use this script.

Start by tracing the non-configure build path and its flags for zlib on Apple Silicon, using ./configure as the stated reference. Confirm where HAVE_UNISTD_H must be passed and verify that the affected functions are defined and the build succeeds without relying on the configure script.

Written by the indexing model from the issue text.