[loumalouomega]

Description

CO_SIM_IO_BUILD_PYTHON_MPI4PY_INTERFACE never added the mpi4py include directory. mpi4py_interface.cpp includes <mpi4py/mpi4py.h>, so the build only worked if the caller passed -isystem <site-packages>/mpi4py/include through CMAKE_CXX_FLAGS. The CI job currently does that as a workaround.

Changes

• When the option is enabled, query the Python interpreter for mpi4py.get_include(). It uses Python_EXECUTABLE and falls back to PYTHON_EXECUTABLE, because pybind11 may set either.
• Add the result to PyCoSimIOMPI_mpi4pyInterface as a SYSTEM include directory.
• Stop configuration with a clear error if mpi4py can't be imported or its include directory doesn't exist.

Testing

• Configured with -DCO_SIM_IO_BUILD_PYTHON=ON -DCO_SIM_IO_BUILD_MPI=ON -DCO_SIM_IO_BUILD_PYTHON_MPI4PY_INTERFACE=ON and no CMAKE_CXX_FLAGS.
• Built PyCoSimIOMPI_mpi4pyInterface successfully.