Only override VTK_GROUP_ENABLE_MPI when DEFAULT.
Turning VTK_GROUP_ENABLE_MPI=YES
when VTK_USE_MPI=ON
disallows valid configurations where only some of the MPI based modules are wanted (ie. -DVTK_GROUP_ENABLE_MPI=DONT_WANT -DVTK_MODULE_ENABLE_VTK_IOParallelXML
).
VTK_GROUP_ENABLE_MPI=DEFAULT
is now WANT
if VTK_USE_MPI=YES
.