build: don't build deps/zlib if --shared-zlib set

Even if the --shared-zlib flag was used, the bundled deps/zlib was still
being compiled into the binary as it was required by the C++ test suite.

PR-URL: https://github.com/nodejs/node/pull/10657
Fixes: https://github.com/nodejs/node/issues/10649


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>