CMake: Skip MPICXX

We do not use the MPI-2 only MPI C++ bindings, but some
implemenations (esp. OpenMPI) still create an internal
dependency on it, which is unnecessary and complicates tooling
and deployments.

Author: ax3l

.github/workflows/tooling.yml

@@ -61,7 +61,7 @@ jobs:
         SOURCEPATH="$(pwd)"
         share/openPMD/download_samples.sh build
         export LDFLAGS="${LDFLAGS} -fsanitize=address,undefined -shared-libsan"
-        export CXXFLAGS="${CXXFLAGS} -fsanitize=address,undefined -shared-libsan -DOMPI_SKIP_MPICXX"
+        export CXXFLAGS="${CXXFLAGS} -fsanitize=address,undefined -shared-libsan"
         cmake -S . -B build               \
           -DopenPMD_USE_MPI=ON            \
           -DopenPMD_USE_PYTHON=ON         \

CMakeLists.txt

@@ -233,6 +233,7 @@ message(STATUS "openPMD-api superbuild: ${openPMD_SUPERBUILD}")
 #     Clang+MPI: Potentially needed MPI::MPI_C targets in the past
 #                (exact MPI flavor & Clang version lost)
 #     BullMPI: PUBLIC dependency to MPI::MPI_CXX is missing in MPI::MPI_C target
+set(MPI_CXX_SKIP_MPICXX ON)  # We do not use MPI-2 only MPI C++ bindings
 set(openPMD_MPI_LINK_C_DEFAULT OFF)
 option(openPMD_MPI_LINK_C  "Also link the MPI C targets" ${openPMD_MPI_LINK_C_DEFAULT})
 mark_as_advanced(openPMD_MPI_LINK_C)