[bugfix] Choose a gcc4.4 compliant header to test for dune-istl

needed for webpage.

(cherry picked from commit e41e47f4)
Signed-off-by: Steffen Müthing <muething@dune-project.org>

m4/dune_istl.m4

@@ -20,5 +20,5 @@ AC_DEFUN([DUNE_ISTL_CHECKS],
 
 AC_DEFUN([DUNE_ISTL_CHECK_MODULE],
 [
-    DUNE_CHECK_MODULES([dune-istl], [istl/istlexception.hh])
+    DUNE_CHECK_MODULES([dune-istl], [istl/solvercategory.hh])
 ])