There is logic in ExternalLibraries/MPI/configure.sh to automatically detect the location of MPI. This code looks for $X/lib/libmpi.a and $X/include/mpi.h for various values of X. This is not a robust way to locate MPI because libmpi.a is not the correct filename on Mac OS. We could make a special case for Mac OS and use ".dylib" instead of ".a", or we could remove the library file from the test, and rely on the header file alone. I prefer the latter.