fortran mpifh: only compile the sizeof lib when necessary

This change was already on master; it was apparently made a long time
ago.  Hence, this commit is not a cherry-pick from master (I didn't
go back to try to find that specific fix); it's a unique commit on
v1.10 to bring this specific section of mpif-h/Makefile.am in-line
with what is on master.

Namely: only create the libmpi_mpifh_sizeof.la if there is a need to.
Otherwise, use AM_CONDITIONAL logic to not build it at all.

Author: jsquyres

ompi/mpi/fortran/mpif-h/Makefile.am

@@ -109,13 +109,13 @@ MOSTLYCLEANFILES = *.mod
 # out this workaround indefinitely.  So Jeff advised Absoft to fix
 # this bug in Sep 2014.
 if OMPI_BUILD_FORTRAN_MPIFH_BINDINGS
-noinst_LTLIBRARIES += libmpi_mpifh_sizeof.la
 
 if BUILD_FORTRAN_SIZEOF
+noinst_LTLIBRARIES += libmpi_mpifh_sizeof.la
 # Do not dist this file; it is generated
 nodist_libmpi_mpifh_sizeof_la_SOURCES = sizeof_f.f90
-endif
 libmpi_mpifh_la_LIBADD += libmpi_mpifh_sizeof.la
+endif
 
 sizeof_pl = $(top_srcdir)/ompi/mpi/fortran/base/gen-mpi-sizeof.pl