fortran sizeof: ensure mpi_sizeof*f90 is not empty

Per http://www.open-mpi.org/community/lists/devel/2015/08/17775.php,
some compilers don't like it when there's a .f90 file that only
contains comments (and no actual Fortran code).  So if OMPI determines
that the Fortran compiler does not support enough Fortran mojo to
support MPI_SIZEOF, generate at least one dummy Fortran subroutine
that can be compiled in an otherwise barren Fortran landscape that is
devoid of life and hope.

ompi/mpi/fortran/base/gen-mpi-sizeof.pl

@@ -1,6 +1,6 @@
 #!/usr/bin/env perl
 #
-# Copyright (c) 2014 Cisco Systems, Inc.  All rights reserved.
+# Copyright (c) 2014-2015 Cisco Systems, Inc.  All rights reserved.
 # Copyright (c) 2015      Research Organization for Information Science
 #                         and Technology (RIST). All rights reserved.
 # $COPYRIGHT$
@@ -218,12 +218,23 @@ sub output_file {
 ! Specifically: we need support for the INTERFACE keyword,
 ! ISO_FORTRAN_ENV, and the STORAGE_SIZE() intrinsic on all types.
 ! Apparently, this compiler does not support both of those things, so
-! this file will be blank (i.e., we didn't bother generating the
-! necessary stuff for MPI_SIZEOF because the compiler doesn't support
+! this file will be (effecitvely) blank (i.e., we didn't bother
+! generating the necessary stuff for MPI_SIZEOF because the compiler
+! doesn't support
 ! it).
 !
 ! If you want support for MPI_SIZEOF, please use a different Fortran
-! compiler to build Open MPI.\n\n";
+! compiler to build Open MPI.
+
+! Dummy subroutine, just so that there is *some* Fortran in this file
+! (some compilers are unhappy if there are no Fortran statements in this
+! file).
+subroutine ompi_sad_panda()
+  implicit none
+
+  print *, 'Open MPI is a sad panda because your Fortran compiler'
+  print *, 'does not support enough Fortran mojo for MPI_SIZEOF'
+end subroutine ompi_sad_panda\n\n";
     }
 
     close(OUT);