cmake configuration failure

```
 CMake Error at SuperBuild/External_SimpleITK.cmake:66 (get_filename_component):
   get_filename_component called with incorrect number of arguments
 Call Stack (most recent call first):
   CMake/SlicerMacroCheckExternalProjectDependency.cmake:69 (include)
   SuperBuild.cmake:264 (SlicerMacroCheckExternalProjectDependency)
   CMakeLists.txt:29 (include)
```
