Linking fails on Windows if other -sys crates link to the system zlib

I'm trying to compile something for Windows (using msvc), but I have a static link version of zlib on my include and lib paths. Other -sys crates link to the system installed zlib, but as libz-sys builds zlib again this leads to the following error:

  = note: liblibz_sys-aa3193a377538516.rlib(inflate.o) : error LNK2005: inflateCodesUsed already defined in libmozjs_sys-a5f33b0cb310126e.rlib(inflate.obj)
          liblibz_sys-aa3193a377538516.rlib(inflate.o) : error LNK2005: inflateValidate already defined in libmozjs_sys-a5f33b0cb310126e.rlib(inflate.obj)
          liblibz_sys-aa3193a377538516.rlib(deflate.o) : error LNK2005: deflateGetDictionary already defined in libmozjs_sys-a5f33b0cb310126e.rlib(deflate.obj)
          liblibz_sys-aa3193a377538516.rlib(adler32.o) : error LNK2005: adler32_z already defined in libmozjs_sys-a5f33b0cb310126e.rlib(adler32.obj)
          liblibz_sys-aa3193a377538516.rlib(crc32.o) : error LNK2005: crc32_z already defined in libmozjs_sys-a5f33b0cb310126e.rlib(crc32.obj)

Is it possible for libz-sys to use the system zlib if it's available? I think the call to zlib_installed should be just made on msvc as well and that's it, but I'm not sure if that's all there is to it.

Start by locating the zlib_installed check and the MSVC-specific build path, then reproduce the duplicate-symbol failure with another -sys crate providing zlib. Done means that an available system zlib is detected on MSVC and libz-sys no longer builds a conflicting copy.