dependencies.rb: Add MPI compiler Requirement

All versions of OS X prior to Lion shipped with some version of Open-MPI, but
without working compiler wrappers for Fortran. Homebrew currently has two
formulae that can supply this software: open-mpi and mpich2.

This commit adds a `MPIDependency` Requirement that can be passed one of four
values when constructed:

    :cc, :cxx, :f90, :f77

This will ensure the `mpi<value>` compiler is available and working. For
example, if `depends_on MPIDependency.new(:cc, :f90)` is used, the Requirement
will search for working `mpicc` and `mpif90` wrappers.

If the required wrappers cannot be found, an error message will be displayed
that prompts the user to install one of the Homebrew formulae that provides
MPI.

For each language passed to a MPIDependency Requirement, environment variables
be will set that point to the compilers.

Signed-off-by: Charlie Sharpsteen <source@sharpsteen.net>

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