ENH: Gate vnl_math.h include in itkMath.h behind ITK_FUTURE_LEGACY_REMOVE

itk::Math no longer uses anything from vnl_math.h. The include remains
for downstream code reaching vnl_math:: transitively through itkMath.h,
and disappears under ITK_FUTURE_LEGACY_REMOVE builds.

Author: hjmjohnson

Modules/Core/Common/include/itkMath.h

@@ -40,7 +40,10 @@
 #endif
 #include "itkMathDetail.h"
 #include "itkConceptChecking.h"
-#include <vnl/vnl_math.h>
+#if !defined(ITK_FUTURE_LEGACY_REMOVE)
+// itk::Math no longer uses vnl_math; kept transitionally for downstream vnl_math:: users.
+#  include <vnl/vnl_math.h>
+#endif
 
 namespace itk::Math
 {