The problem with GCC version is that CMake does not always use PATH to find compilers. To fix this, set the CC and FC environment variables to the compilers you want to use and then re-run the build script for MPI.jl. Note that you should use the MPI compiler wrappers for the compilers loaded with module load. I usually do
export CC=`which mpicc`
export FC=`which mpif90`
I’m not sure if this is the problem with MPI_COMM_NULL not being defined, but let’s start here an see what happens.
Also, can you post the contents of ~/.julia/packages/MPI/U5ujD/deps/build.log? I think that is where the output from the MPI.jl build script was redirected to.