Building with MPI using oneAPI 2025.1's Intel MPI Fails

[BenBrock]

When building using oneAPI 2025.1's Intel MPI, I get the following failure from CMake:

-- Could NOT find MPI_CXX (missing: MPI_CXX_WORKS)
CMake Error at /data/nfs_home/bbrock/pkg/cmake-3.30.1-linux-x86_64/share/cmake-3.30/Modules/FindPackageHandleStandardArgs.cmake:233 (message):
  Could NOT find MPI (missing: MPI_CXX_FOUND CXX)
Call Stack (most recent call first):
  /data/nfs_home/bbrock/pkg/cmake-3.30.1-linux-x86_64/share/cmake-3.30/Modules/FindPackageHandleStandardArgs.cmake:603 (_FPHSA_FAILURE_MESSAGE)
  /data/nfs_home/bbrock/pkg/cmake-3.30.1-linux-x86_64/share/cmake-3.30/Modules/FindMPI.cmake:1841 (find_package_handle_standard_args)
  cmake/utils.cmake:121 (find_package)
  CMakeLists.txt:68 (setup_runtime_backends)

This is invoking CMake like such:

CXX=icpx CC=icx cmake .. -DCMAKE_INSTALL_PREFIX=${ISHMEM_INSTALL_DIR} -DCMAKE_BUILD_TYPE=Release -DENABLE_MPI=ON -DMPI_DIR=${MPI_DIR} -DENABLE_OPENSHMEM=ON -DSHMEM_DIR=${SOS_INSTALL_DIR}

Switching to the "C" instead of "CXX" component of MPI in utils.cmake fixes the issue.

I don't believe Intel SHMEM uses any of the deprecated MPI-2 C++ API, so I believe C is the correct component here.

[BenBrock]

(And just to avoid confusion, the utils.cmake in this mirror links with MPI differently; it's the innersource mirror that has the issue.)