[cmake] do not export dune-istl version but use module local detection.

According to @rolk defines should only be exported in
cmake/Modules/<module>-prereqs.cmake if they are used in the
headers. This is not the case for the dune-istl version macros and
therefore we use opm_need_version_of in the config_hook in the top
level CMakefile with this patch.

CMakeLists.txt

@@ -46,6 +46,7 @@ include (CMakeLists_files.cmake)
 
 macro (config_hook)
 #	opm_need_version_of ("dune-common")
+  opm_need_version_of ("dune-istl")
 	list (APPEND ${project}_CONFIG_IMPL_VARS
 		HAVE_DUNE_ISTL
 		)

cmake/Modules/opm-core-prereqs.cmake

@@ -3,9 +3,6 @@
 
 # defines that must be present in config.h for our headers
 set (opm-core_CONFIG_VAR
-  DUNE_ISTL_VERSION_MAJOR
-  DUNE_ISTL_VERSION_MINOR
-  DUNE_ISTL_VERSION_REVISION
 	HAVE_ERT
 	HAVE_SUITESPARSE_UMFPACK_H
 	)