curl-sys' vendoring depends on libz-sys vendoring

When `libz-sys` fails to find `zlib`, it falls back to building it from source.

However, when `curl-rust` vendors `libcurl` (when it falls back), it transitively depends on either `zlib` being present in system include paths **or** `libz-sys` vendoring it into the same paths:

https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L57-L59

`libz-sys` recently attempted to use `pkg-config` on Windows, which broke `curl-sys` using an MSVC toolchain (from inside an MSYS2 environment, presumably _without_ `vcpkg`), because it would no longer need to build `zlib` from source: https://github.com/rust-lang/libz-sys/issues/143

However, because `curl-sys` does not support `pkg-config` (#486), on Windows:

* on an MSVC **host** and MSVC **target**, it will attempt to find `libcurl` with `vcpkg` (#509 would slightly improve this)
* otherwise, it just builds `libcurl` from source

When building from source, `curl-sys` generates a `pkg-config` file, but doesn't add any linkage or include path information for `zlib`:

https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L98-L100

...and enables `zlib` support:

https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L127

...it might get something from `vcpkg` if built on an MSVC **host** (because `build.rs` cfg directives are based on the host, not the target):

https://github.com/alexcrichton/curl-rust/blob/ff6ad21cc1034826b2ab3f8be0653d8c446e1bdc/curl-sys/build.rs#L296-L299

...but otherwise this means `zlib.h` needs to be in `$OUT_DIR/include` (which it gets from `libz-sys`' vendoring) or in the system include paths (which it gets on most non-Windows platforms).

When vendoring `libcurl`, `curl-sys` should find `zlib` properly (ideally, the same way as `libz-sys` does).

It would be better if you needed to explicitly enable vendoring in this library, because while "silent fallback" can be helpful, it leads to surprising behaviour like this, and makes it more difficult to audit your dependencies.