I don't think this was the problem. We call updateCompilers() after the install, so it should override anything the user puts in.

If I understand Barry correctly, it's that people who specify this: $ ./configure CC=mpicc --CXX=mpicxx --download-mpich clearly do not understand the --download-mpich option and should be told: ERROR: User needs to learn MPI; either use the cluster's custom built mpi XOR use --download-mpich because if configure && make succeed, then they will irrevocably use the mpiexec or mpicc in their $PATH and then use such great lines as "I have no idea what MPI is!" or "I just want to compile and run on my cluster! Why won't you let me do that?!"