I started a build with CMake 3.16.2 and it failed:
CUSTOMBUILD : error : Target (for copy_if_different command) "D:/D/S4R3/Slicer-build/lib/Slicer-4.11/cli-modules/Release" is not a directory. [D:\D\S4R3\Slicer-build\Base\QTCLI\Testing\InstallPyCLITest4.vcxproj] [D:\D\S4R3\Slicer.vcxproj]
The complete build environment, all tools and options were exactly the same, I just updated CMake from 3.14.3 to 3.16.2. It is either a regression in CMake (which is very common - so far something broke in every major CMake release) or somehow we use CMake incorrectly. As described in the other thread, the problem is that a parallel build is performed by default (and a file is attempted to be copied to a folder that does not exist yet), even though parallel build is not explicitly requested.
It would be interesting to find out why build is not sequential anymore, but it would be probably also useful to allow parallel builds by creating the output directory.