Properly display name of project with dependency found on the system.

Fix type introduced by commit 74d440c5.

See #418

CMake/ctkMacroCheckExternalProjectDependency.cmake

@@ -495,7 +495,7 @@ macro(superbuild_include_dependencies)
   if(NOT "${${proj}_DEPENDENCIES}" STREQUAL "")
     set(_msg "${proj}[OK]")
     if(${SUPERBUILD_TOPLEVEL_PROJECT}_USE_SYSTEM_${proj})
-      set(_msg "${_ok_message} (SYSTEM)")
+      set(_msg "${_msg} (SYSTEM)")
     endif()
     superbuild_message(${proj} ${_msg})
   endif()