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.
separate out cray pmi config

The mixing of the Slurm PMI and Cray PMI configure was getting
messy and dangerous - developers working on Slurm PMI often don't
have access to Cray PMI, etc.

This mod pulls out the Cray PMI configure into a separate m4 file.
Cray pmi is now configured as follows:

1) on Cray CLE 5 and higher, Cray PMI is auto detected.  pkg-config
   is used to resolve the necessary CPP flags, link flags and libs,
   etc.  Nothing needs to be added to the configure line to pick up
   Cray PMI.

2) on legacy Cray CLE 4 systems with PMI 4.X, Cray PMI is also
   auto detected.

3) on legacy Cray CLE 4 systems with PMI 5.X Cray PMI can't be auto-detected
   owing to changes in the PMI pkg-config file which result in pkg-config
   returning an error owing to a dependency of PMI on newer versions of ALPS
   installs that are not present on CLE 4. So, for those falling in to this
   situation, the --with-cray-pmi=(DIR) method needs to be used.

   DIR specifies the Cray PMI install directory.  The configure file looks
   for required alps libraries first in /usr/lib/alps, then in
   /opt/cray/xe-sysroot/default/usr/lib/alps.