ENH: Prevent header generation when unchanged.
ENH: Prevent header generation when unchanged.

Added CMake logic to prevent the regeneration of the headers
and .i files in the case when the list have not actually changed.
This should prevent unnecessary rebuilds after running cmake,
but still should update the .h and .i files when new filters
are added via .json files.

SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging,SimpleITK/Staging