Instead of silenty failing and resulting in broken builds/packages.
The property is only available since CMake 3.19, so limit the feature to
that.
Change-Id: I82b06e78540e81a809f7fdfe130648becfc9672e
Reviewed-by: Cristian Adam <cristian.adam@qt.io>
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>