Update zlib dep to zlib 1.2.12 and make Qt use it
The update is needed to fix a security vulerability:
https://ubuntu.com/security/CVE-2018-25032
On Linux we use system-provided version of zlib, which has been updated yesterday, so we don't add that into the deps.
From now on, Qt stops using embedded zlib and switches to the system provided one on all the platforms.
Edited by Dmitry Kazakov