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 raising the issue.

Fixes open-mpi/ompi#259.

(cherry picked from commit open-mpi/ompi@6164dc4960805b5502f86f9a2dccc5fc557e44d6)