Some of the macros used are only available for CMake 2.8.6 and higher. Bumped...

Some of the macros used are only available for CMake 2.8.6 and higher. Bumped required CMake version

[[Imported from SVN: r1727]]

CMakeLists.txt

@@ -2,7 +2,7 @@
 project("dune-istl" C CXX)
 
 # general stuff
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.6)
 
 #find dune-common and set the module path
 find_package(dune-common)