Fix for building with system MPI

Nick Leafrequested to merge
nleaf/common-superbuild:system-mpi-fix into master

Fixes a problem encountered when building with system MPI in a Docker container by adding a line to pull in the location of the MPI Fortran compiler (in addition to the C and CXX compilers that were already there).

Backport: paraview/release

Edited by Ben Boeckel

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