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.

config/ompi_setup_fc.m4

@@ -13,7 +13,7 @@ dnl                         All rights reserved.
 dnl Copyright (c) 2007      Los Alamos National Security, LLC.  All rights
 dnl                         reserved. 
 dnl Copyright (c) 2007      Sun Microsystems, Inc.  All rights reserved.
-dnl Copyright (c) 2009-2013 Cisco Systems, Inc.  All rights reserved.
+dnl Copyright (c) 2009-2014 Cisco Systems, Inc.  All rights reserved.
 dnl $COPYRIGHT$
 dnl 
 dnl Additional copyrights may follow
@@ -131,23 +131,26 @@ AC_DEFUN([OMPI_SETUP_FC],[
            [AC_FC_SRCEXT(f)
             AC_FC_SRCEXT(f90)])
 
-    # Per #1982, on OS X, we may need some esoteric linker flags in
-    # the wrapper compilers.  Assume that we need it for both F77 and
-    # FC flags (note that in an upcoming update where there will only
-    # be one Fortran compiler, anyway).
+    # Per trac #1982, on OS X, we may need some esoteric linker flags
+    # in the wrapper compilers.  However, per
+    # https://github.com/open-mpi/ompi/issues/259, we need to use
+    # -Wl,-flat_namespace when *building* the library (and
+    # -Wl,-commons,use_dylibs isn't quite sufficient).
     AS_IF([test $ompi_fc_happy -eq 1],
           [AC_MSG_CHECKING([to see if Fortran compilers need additional linker flags])
            case "$host" in
            *apple-darwin*)
-               # Test whether -Wl,-commons,use_dylibs works; if it
-               # does, use it.
+               # Test whether -Wl,-flat_namespace works; if it does,
+               # both use it to build the libraries, and also put it
+               # in the wrapper compiler LDFLAGS.
                LDFLAGS_save=$LDFLAGS
-               LDFLAGS="$LDFLAGS -Wl,-commons,use_dylibs"
+               LDFLAGS="$LDFLAGS -Wl,-flat_namespace"
                AC_LANG_PUSH([Fortran])
                AC_LINK_IFELSE([AC_LANG_SOURCE([[program test
   integer :: i
 end program]])],
-                              [OMPI_FORTRAN_WRAPPER_FLAGS="-Wl,-commons,use_dylibs"
+                              [LDFLAGS_save=$LDFLAGS
+                               OMPI_FORTRAN_WRAPPER_FLAGS="-Wl,-flat_namespace"
                                OPAL_WRAPPER_FLAGS_ADD([FCFLAGS], [$OMPI_FORTRAN_WRAPPER_FLAGS])],
                               [OMPI_FORTRAN_WRAPPER_FLAGS=none])
                AC_LANG_POP([Fortran])