Fix the LIBZ macro on VC config targets

If zlib-dynamic was given but not --with-zlib-lib, LIBZ was defined to
the empty string.  Instead, give it the default "ZLIB1".

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1772)
(cherry picked from commit 111b234c)