Allow checks for the dune-subgrid module. dune-subgrid is a dune extension developed

by Carsten Gräser and myself which allows computations on subsets of grids.  Since
it is not a core module there should be no reference to it in dune-common, however
as long as Bug 230 is not fixed there is no way around it.

[[Imported from SVN: r4893]]

m4/dune.m4

@@ -235,6 +235,8 @@ AC_DEFUN([DUNE_CHECK_DISPATCH],[
            DUNE_CHECK_MODULES([dune-istl], [istl/allocator.hh])],
          [$1], [dune-disc],[
            DUNE_CHECK_MODULES([dune-disc], [disc/shapefunctions/lagrangeshapefunctions.hh], [Dune::LagrangeShapeFunctions<double[,]double[,]3>::general])],
+         [$1], [dune-subgrid],[
+           DUNE_CHECK_MODULES([dune-subgrid], [subgrid/subgrid.hh])],
          [AC_MSG_ERROR([Unknown module $1])])
 ])