COMP: Fix Superbuild extension configuration not having git/svn in the path

This commit fixes the following error:

[Subversion_SVN_EXECUTABLE-NOTFOUND]
Call Stack (most recent call first):
C:/D/N/Slicer-1/Extensions/CMake/SlicerEnableExtensionTesting.cmake:21 (include)
C:/D/N/Slicer-1-build/Slicer-build/UseSlicer.cmake:404 (include)
CMakeLists.txt:20 (include)

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25044 3bd1e089-480b-0410-8dfb-8563597acbee