ADD_ELXCOMPONENT( GradientDifferenceMetric # Was OFF by default before elastix 5.0.1
 elxGradientDifferenceMetric.h
 elxGradientDifferenceMetric.hxx
 elxGradientDifferenceMetric.cxx
 itkGradientDifferenceImageToImageMetric2.h
 itkGradientDifferenceImageToImageMetric2.hxx)