mpi: include VTK's MPI header wrapper

CMakeLists.vtk.txt

 vtk_module_install_headers(
   SUBDIR      "vtkdiy2"
   DIRECTORIES "include")
+
+set(diy_has_mpi 0)
+if (TARGET VTK::mpi)
+  set(diy_has_mpi 1)
+endif ()
+
+vtk_module_definitions(VTK::diy2
+  INTERFACE
+    "DIY_HAS_MPI=${diy_has_mpi}")

include/diy/mpi/config.hpp

@@ -19,7 +19,7 @@
 /// only included for the library's compilation units.
 #ifdef DIY_HAS_MPI
 #  if DIY_HAS_MPI
-#    include <mpi.h>
+#    include <vtk_mpi.h>
 #  else
 #    include "no-mpi.hpp"
 #  endif