That is required for qtopenapi builds that would be enabled in the
follow-up patch.
The zlib library is already built after commit
97e80c1538. That commit also sets the
ZLIB_PATH_{arch} environment variables. So, we only need to set
ZLIB_ROOT to point to the proper zlib installation.
This patch does not cover llvm and mingw configurations, because we
need to builds zlib using these compilers first.
Task-number: QTBUG-138968
Change-Id: I5d5ad754d13bef9fc6184b41823a42d98863991a
Reviewed-by: Toni Saario <toni.saario@qt.io>
aecfc7e23b