The CMakeLists file for the static zlib checks for presence of
unistd.h, but it doesn't properly define HAVE_UNISTD_H if it's found.
This change adds the necessary preprocessor definition if unistd.h is
found.
Upstream zlib handles this with by configuring zconf.h with CMake:
cacf7f1d4e/zconf.h.cmakein (L11)
Dolphin's static version of zlib doesn't do this, which is why setting
Z_HAVE_UNISTD_H in zlib's CMakeLists.txt isn't enough.
This probably wasn't noticed since because most *nix systems will use
the shared zlib. Force use of the static zlib (comment out
find_package(ZLIB) in the root CMakeLists.txt) and you'll see implicit
function declaration warnings during its compilation.
24226419ed