fortran: only build the back-compat F08 symbols when necessary

Previous code mistakenly always built the F08 back-compat MPI_SIZEOF symbols in libmpi_mpifh.so.  Change it to build the F08 back-compat symbols only if we're also building the mpi_f08 module (i.e., the Fortran compiler supports all the stuff we need for the F08 module).