In particular, Qt targets extended with other qt targets
(eg. Qt::VulkanSupport or Qt::LinuxAccessibilitySupport)
after the first add_qt_module were not taken into account when generating
Depends files.
Note that this patch updates the minimum required version
to CMake 3.15
Change-Id: I747deedd4d59e385876bc1a834ef9bdb6078911b
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>