[petsc-dev] Install location for MPIUNI mpi.h

[Jed Brown]

It is currently installed to include/petsc/mpiuni/mpi.h and petscsys.h
includes it as <mpi.h>, which means that users of MPIUNI need to put
-I/prefix/include/petsc/mpiuni in their command lines.  Matt and I agree
that this is bad.  We disagree on the solution.

He wants to install it to /prefix/include/mpi.h as though the user had
written --download-mpich.  This would conflict if a user later installs
a real MPI to that location.

I would rather that petscsys.h include <petsc/mpiuni/mpi.h> because it
can't be used without PETSc and nobody who ever wrote #include <mpi.h>
in their own code will be happy if they get MPIUNI.