BLD: Force defining the mdspan parenthesis operator even with C++ >= 23. (#22588)

This allows to keep the same code, independently of the compiler C++
dialect being used. The default behavior of kokkos mdspan is the
following:
 - if __cpp_multidimensional_subscript is defined (ie C++ >= 23), then
   enable by default the square bracket operator to access elements, and
   disable by default the parenthesis operator.
 - otherwise (C++ < 23), disable the square bracket operator which
   cannot work, and replace it by the parenthesis operator instead.

The current xsf code always uses the parenthesis operator. To avoid
having to always write the two possibilities, force the kokkos mdspan
implementation to always define the parenthesis operator for now.

Signed-off-by: Romain Geissler <[email protected]>

Author: Romain-Geissler-1A

include/xsf/numpy.h

@@ -19,6 +19,9 @@
 
 #include "dual.h"
 #include "error.h"
+// Force defining the parenthesis operator even when compiling with a compiler
+// defaulting to C++ >= 23.
+#define MDSPAN_USE_PAREN_OPERATOR 1
 #include "third_party/kokkos/mdspan.hpp"
 
 /* PyUFunc_getfperr gets bits for current floating point error (fpe) status codes so we