Warning message configuring with kokkos 3.4.1 on Perlmutter about omega_h

[cwsmith]

Thank you for reporting this. Apparently, the Cray compiler wrappers and CMake made a blood pact to set MPI_FOUND=true and not set any of the variables folks rely on to compile and link against MPI.

Specifically, for packages not using the Cray compiler wrappers (this is what omega_h does), this NERSC page suggests adding a bunch of logic after find_package(MPI) to define an foo-mpi target which contains the needed flags.

[zhangchonglin]

@cwsmith: actually, just saw this on Summit too with gcc 7.5.0 and spectrum-mpi/10.4.0.3-20210112:

-- Kokkos_VERSION: 3.1.1
-- Omega_h_USE_libMeshb: OFF
-- Omega_h_USE_Gmsh: OFF
-- Omega_h_USE_Gmodel: OFF
-- Omega_h_USE_SEACASExodus: OFF
-- Omega_h_USE_pybind11: OFF
-- Omega_h_USE_CUDA: on
-- Omega_h_USE_MPI: on
-- Omega_h_USE_MPI: on
-- MPI_PREFIX: 
CMake Warning at CMakeLists.txt:102 (message):
  MPI Libraries not found, there may be linking errors! Try enabling shared
  libraries '-DBUILD_SHARED_LIBS=ON'.