build: export zlib symbols on Windows

Base the generated openssl.def on existing zlib.def. We cannot specify
more than one DEF file per executable so we need to merge the two DEF
files to expose both OpenSSL and Zlib functionality to addons.

If OpenSSL is not used, link against zlib.def itself.

PR-URL: https://github.com/nodejs/node/pull/7983


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>