Do build zlib as static 3rdparty library. This makes it easier to
disable warnings.
Pick-to: 6.3
Change-Id: I1db331b671b64e68d81c56b0df337983c3bbe7fa
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>