SuperBuild: itk forbid download option to avoid download fftw if not found

SuperBuild/CMake/External_itk.cmake

@@ -156,6 +156,7 @@ ExternalProject_Add(ITK
   -DITK_USE_SYSTEM_ZLIB:BOOL=ON
   -DITK_USE_SYSTEM_TIFF:BOOL=ON
   -DITK_USE_SYSTEM_PNG:BOOL=ON
+  -DITK_FORBID_DOWNLOADS:BOOL=ON
   ${ITK_SB_CONFIG}
   DEPENDS ${ITK_DEPENDENCIES}
   CMAKE_COMMAND ${SB_CMAKE_COMMAND}