COMP: Update imstk_add_external_project to keep track of the project var

This ensures external project like the one in the SlicerIMSTK extension
will have their project variable value restored after the call to
"ExternalProject_Include_Dependencies" returns.

CMake/Utilities/imstkAddExternalProject.cmake

@@ -65,9 +65,9 @@ macro(imstk_add_external_project extProj)
   #-----------------------------------------------------------------------------
   # Solve dependencies
   #-----------------------------------------------------------------------------
-  set(PROJECT_VAR_TEMP ${extProj})
+  set(extProj ${extProj})
   ExternalProject_Include_Dependencies( ${extProj}
-    PROJECT_VAR PROJECT_VAR_TEMP
+    PROJECT_VAR extProj
     EP_ARGS_VAR ${extProj}_EP_ARGS
     DEPENDS_VAR _imstk_add_ep_${extProj}_DEPENDENCIES
     USE_SYSTEM_VAR USE_SYSTEM_${extProj}