[cmake] yet another macro switch default warning fix

config.h.cmake

@@ -122,6 +122,10 @@
 # define HAVE_ALBERTA 0
 #endif
 
+#ifndef HAVE_GRAPE
+# define HAVE_GRAPE 0
+#endif
+
 /*** End: Silence implicitly False evaluation of undefined macro warnings ****/
 
 /* end dune-stuff */