Another ACX_MPI fallback

[roystgnr]

We currently end up in an inconsistent state when configuring against the repo libpetsc3.4.2-dev on Ubuntu 14.10 - PETSc is there, and configured with MPI, but doesn't define MPIEXEC, so we assume it's configured in serial, and we don't try to detect MPI.

This PR fixes the configure for me, by trying ACX_MPI in that case. It's possible that this PR will break cases in which the user has an autodetectable MPI but wants to configure against a mpiuni PETSc, but in those cases "--with-mpi=false" ought to be an adequate workaround.

[roystgnr]

Forgot to mention: thanks go to Zhenyu (more id omitted since I don't know his github username and I want to spare his email address from spammers) for finding this problem. I'm running on Ubuntu 14.10 myself but haven't been using the default repo PETSc for a long time.

[jwpeterson]

So are we wrong to be grepping for MPIEXEC in petscvariables as a way to snoop MPI? It looks like we've been doing it that way since time immemorial...

I can definitely see why Ubuntu can't simply tie its PETSc dpkg to a single mpiexec since it has no idea what a user might install...

[jwpeterson]

I don't suppose Ubuntu's petscvariables file provides MPI_INCLUDE, MPICXX_SHOW, or MPICC_SHOW variables either?

[roystgnr]

Nothing in petscvariables, or in the rest of /usr/lib/petsc/conf/* as far as grep can tell.

[roystgnr]

I can't imagine that merely omitting config variables produces an MPI-independent PETSc, though. IIRC the MPI standard doesn't specify enough of an ABI to make that possible. I guess I'll try swapping between openmpi and mpich2 and find out for myself, though.

[moosebuild]

Results of testing ecc0616 using libmesh_PR_test recipe:

Passed on: linux-gnu

View the results here: https://www.moosebuild.com/view_job/13917

[moosebuild]

Results of testing ecc0616 using libmesh_PR_test_dbg recipe:

Passed on: linux-gnu

View the results here: https://www.moosebuild.com/view_job/13918

[roystgnr]

Trying to use Ubuntu PETSc with mpich2 ends up adding mpich2 compile/link flags via ACX_MPI autodetection and openmpi compile/link flags via PETSCINCLUDEDIRS/PETSCLINKLIBS. I'm not going to bother testing that case. This works for the Ubuntu PETSc+openmpi case, though, so ready to merge.