Better detection for user-requested Fortran bindings.

As discovered by the nightly build, r30379 broke the case where
configure does not find a fortran compiler, and no form of
--enable-mpi-fortran was specified.

This commit specifically calls out the difference between a user
specifying that they want Fortran bindings and the default "try to
compile all the Fortran bindings" cases.

cmr=v1.7.4:ticket=4162