boost_mpi tries to find the C++ MPI API

[aminiussi]

boost_mpi is only based on the C MPI API. Yet the the generated cmake code tries to import the C++ API, bringing in more stuff than necessary. Along with the associated problems:

CMake Error in /ccc/work/cont003/gen7233/miniussa/views/fargo/fargOCA/icc/rel/CMakeFiles/CMakeTmp/CMakeLists.txt:
  Imported target "MPI::MPI_CXX" includes non-existent path

[pdimov]

MPI_CXX is not the C++ API. See https://cmake.org/cmake/help/latest/module/FindMPI.html:

The difference between CXX and MPICXX is that CXX refers to the MPI C API being usable from C++, whereas MPICXX refers to the MPI-2 C++ API that was removed again in MPI-3.