Fix unnecessary search path warning on macOS

[madsmtm]

The linker on macOS warns on unused search paths, and concretely, when compiling zlib-ng via this crate, it warns with:

ld: search path 'target/debug/build/libz-sys-3eb8b95c9c44bf93/out/lib64' not found

This warning has previously been hidden away, but rustc wants to surface them by default at some point.

So let's avoid the warning by setting CMAKE_INSTALL_LIBDIR explicitly, and then only passing a single search path to the linker.

Can be tested on macOS with:

$ RUSTFLAGS="-Wlinker-messages" cargo +nightly test --no-default-features --features zlib-ng

[Byron]

Thanks a lot! I validated he warning, and that it doesn't occur with this PR.