build: do not define ZLIB_CONST

This define is not available in zlib prior to version 1.2.5.2. See
https://github.com/nodejs/node/issues/9110 for details. Workaround the
build breakage reported by casting away const in src/inspector_agent.cc
instead.

PR-URL: https://github.com/nodejs/node/pull/9122


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Rod Vagg <rod@vagg.org>