[fix,cmake] fix the cmake magic to enable backwards compatibility for the category interface

cmake/modules/DuneIstlMacros.cmake

@@ -15,4 +15,4 @@ include(AddSuiteSparseFlags)
 
 # enable / disable backwards compatibility w.r.t. category
 set(DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE 1
-  "Enable/Disable the backwards compatibility of the category enum/method in dune-istl solvers, preconditioner, etc. '1'")
+  CACHE BOOL "Enable/Disable the backwards compatibility of the category enum/method in dune-istl solvers, preconditioner, etc. '1'")

config.h.cmake

@@ -71,6 +71,9 @@
 /* Define to the revision of dune-istl */
 #define DUNE_ISTL_VERSION_REVISION ${DUNE_ISTL_VERSION_REVISION}
 
+/* Enable/Disable the backwards compatibility of the category enum/method in dune-istl solvers, preconditioner, etc. */
+#cmakedefine DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE @DUNE_ISTL_SUPPORT_OLD_CATEGORY_INTERFACE@
+
 /* end dune-istl
    Everything below here will be overwritten
 */