[Cmake] Skip MPI cxx link also for mpich

[tdavidcl]

Shamrock/cmake/ShamConfigureMPI.cmake

Line 18 in f006bd8

set(SHAM_CXX_MPI_FLAGS "-DOMPI_SKIP_MPICXX")

should be more like

find_package(MPI COMPONENTS C)
  if(MPI_FOUND)
    # Workaround for various MPI implementations forcing the link of C++ bindings
    add_definitions(-DOMPI_SKIP_MPICXX -DMPICH_SKIP_MPICXX)

    list(APPEND _components MPI)
  endif()

cf https://docs.alcf.anl.gov/aurora/workflows/adios/#mixed-c-and-python-hello-world-example