Build dependencies are too specific to mpich and OpenMPI

[bjpop]

It was observed on a Reddit thread that haskell-mpi has become build dependent on mpich2 or OpenMPI:

http://www.reddit.com/r/haskell/comments/1amua2/using_haskellmpi_as_language_agnostic_ipc/

In the .cabal file we have:

if flag(mpich14)
extra-libraries: mpich, opa, mpl
else
extra-libraries: mpi, open-rte, open-pal

This causes trouble if you try to build with some other version of MPI.

It would be good to have a way to build the library with a minimal dependency on libmpi and mpi.h. Features which are specific to particular MPI implementations should be optional.