Fix for building mpi4py on Windows with msmpi

For mpi4py to work correctly with msmpi, it is necessary to define MSMPI_VER when mpi4py.MPI is compiled.

This MR adds the following internal variable to the cache (e.g. for msmpi 10.1, 0xa01):

//Hexadecimal msmpi version (with 0x) for mpi4py
VTK_MPI4PY_MSMPI_VER:INTERNAL=0xa01

See third-party/mpi4py!6 (merged).

Backport: release