Add ZLIB_DIR config option, and don't vendor zlib or zlib-ng automatically

libz-sys should have a configuration option like ZLIB(_LIB|_INCLUDE)_DIR which allows you to explicitly point at which zlib to use on all platforms, similar to what openssl-sys does with OPENSSL(_LIB|_INCLUDE)_DIR (documentation).

If those environment variables are unset, then libz-sys should continue to use vcpkg, pkg-config or system paths to find zlib.

Additionally, vendoring should be behind a feature flag, disabled by default, so that libz-sys failng to find zlib is a hard error by default:

• While auto-vendoring makes libz-sys build in more scenarios with zero configuration, it leads to surprising behaviour, and makes it more difficult to track down your dependencies. Again, this is something that openssl-sys does well.

• There's also no way to reliably force the library to be vendored, except by setting the static option - which is itself implemented incorrectly, as it never checks for a system-installed static zlib except with vcpkg.

These changes would require a major version bump, as they're SemVer incompatible changes, and will likely break some environments until they can be fixed up.

Some related issues include:

• #143 (caused by curl-sys' auto-vendoring having a transitive dependency on libz-sys building zlib from source on Windows; which was only noticed when libz-sys improved its dependency management so it didn't auto-vendor)
• #132 (cross compiling not using system zlib or correct flags)
• #110 (wants to use pkg-config with zlib-ng, but there's no controls)
• #109 (zlib-ng build process tries to install to /usr/local as part of a fall-back)
• #56 (statically linking two different versions of zlib doesn't work)

#83 has a similar request to disable auto-vendoring for zlib-ng.

Start with build.rs, especially the existing static option and the vcpkg, pkg-config, and system discovery paths. Compare the referenced openssl-sys build logic and configuration documentation. Done means explicit ZLIB(_LIB|_INCLUDE)_DIR handling across platforms, opt-in rather than default vendoring, and a hard error when no usable zlib is found.