[cray-mpich] add support by EnvVar and demote CXX types to C

[HugoStrand]

This pull requests makes the modifications required to support Cray/HPE MPICH 8.1.18.

This (most recent, June 2022) version of MPICH does not support the C++ types, hence the change demotes the C++ types to the C equivalents, i.e.

MPI_CXX_BOOL --> MPI_C_BOOL
MPI_CXX_DOUBLE_COPLEX --> MPI_C_DOUBLE_COMPLEX

Additionally the MPICK environment is detected by adding the environment variable CRAY_MPICH_VERSION to the list of "accepted environment variables".

The type demotion does not affect the reduction etc since the memory footprint is the same.

Please consider merging.

(I have also added a simple mpi_cplx test that broadcasts a complex. This shows the issue with the unsupported MPI_CXX_DOUBLE_COMPLEX types.)

[Wentzell]

Thank you @HugoStrand for looking into these compatibility issues.
I don't see a Problem in using the _C_ variables over the _CXX_.
Were you able to build and pass tests for TRIQS/triqs with these changes?