Build src/port/dirmod.c only on Windows.

Since commit ba7c5975ad, port/dirmod.c
has contained only Windows-specific functions.  Most platforms don't
seem to mind uselessly building an empty file, but OS X for one issues
warnings.  Hence, treat dirmod.c as a Windows-specific file selected
by configure rather than one that's always built.  We can revert this
change if dirmod.c ever gains any non-Windows functionality again.

Back-patch to 9.4 where the mentioned commit appeared.

configure

@@ -12083,6 +12083,12 @@ esac
 fi
 
 
+  case " $LIBOBJS " in
+  *" dirmod.$ac_objext "* ) ;;
+  *) LIBOBJS="$LIBOBJS dirmod.$ac_objext"
+ ;;
+esac
+
   case " $LIBOBJS " in
   *" kill.$ac_objext "* ) ;;
   *) LIBOBJS="$LIBOBJS kill.$ac_objext"