Add defines that are used in compilation modules

This define is used in a opm/*.c* file, but only there, and should
therefore be in config.h, but not added to the list that only clients
must also provide.

CMakeLists.txt

@@ -40,6 +40,9 @@ include (CMakeLists_files.cmake)
 
 macro (config_hook)
 #	opm_need_version_of ("dune-common")
+	list (APPEND ${project}_CONFIG_IMPL_VARS
+		HAVE_DUNE_ISTL
+		)
 endmacro (config_hook)
 
 macro (prereqs_hook)