Create null rule for generating .mod files.

Fortran object files depend upon the module files rather than other object files.  This
is convenient as gfortran, for example, can then avoid cascade compilation automatically.
However, there was a bug.  If a module  interface changed, then because Make didn't have
a rule for generating the .mod file, source files which use that module might not
necessarily be required to be rebuilt as Make could assume the .mod file would not change
during its run.  This could be worked around by rerunning Make (which would then notice
the out-of-date object files compared to the .mod files and rebuild them).  A null rule
for the .mod files allow prerequisite chaining to occur and hence this case to be
detected. (See http://www.gnu.org/software/make/manual/html_node/Empty-Recipes.html.)