Misdetection of MPI

[roystgnr]

I could have sworn we had these synched up properly now, but I've got a system where libMesh manages to autodetect MPI but TIMPI fails. TIMPI tries the PETSc-snooped flags, and for whatever reason with this PETSc module it finds include flags but not linker flags. Then when it tries the include flags with CXX=g++ and without linker flags the compilation fails, and TIMPI incorrectly disables MPI rather than trying with mpicxx or mpic++.

This isn't an urgent issue (configuring with CXX=mpicxx works fine) but we should look into it after we're done with the m4 repository refactoring.