Updated zlib to 1.2.8.
Cut down on the use of ZLIB_LIBRARY/ZLIB_INCLUDE_DIR.

They're not listed in the documentation for the FindZLIB module,
so we shouldn't use them unless necessary.
Set minimal zlib version to 1.2.3.

Rationale: 1.2.3 was a security update, and we should avoid using
versions with known security vulnerabilities.
Merge pull request #1328 from SpecLad:zlib-update