fortran: fix linking with Fortran sentinel values

In OMPI 1.8 and 1.8.1, we didn't allow gfortran to use the ignore-TKR
style "mpi" Fortran module (which is written in Fortran).  Instead,
gfortran used the TKR style "mpi" module, which is written in C.

In OMPI 1.8.2, we removed the gfortran restriction (for suitably
modern gfortran -- i.e., >= 4.9).  This allowed gfortran to use the
ignore-TKR style "mpi" module, but exposed a bug where MPI Fortran
sentinel values wouldn't be linked properly on OS X.  The solution is
to use ```-Wl,-flat_namespace``` (see the OS X ld(1) man page for
details) when both building the libraries and in the wrapper
compilers.

Thanks to Github user yafshar for rasising the issue.

Fixes #259.