Regarding the instructions on how to build an extension (Extensions — 3D Slicer documentation) setting Slicer_DIR to the path c:/D/S4D/Slicer-build is an example path. Based on reading your other posts it appears you have built Slicer in a c:D/S4R directory so your Slicer_DIR is at c:/D/S4R/Slicer-build.
The CMake variable EXTENSION_WC_REVISION is empty ! error is due to building an extension that is not under revision control. You can choose to either build the extension from a git version-controlled folder or specify the extension revision manually as described here.
While obviously developers must always use revision control, Slicer project should probably not enforce this as a mandatory rule. Therefore, I’ve submitted a pull request that changes the build error to just a warning: