[Merge][CMake] Merged revisions : 1727

Revision  r1727
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Some of the macros used are only available for CMake 2.8.6 and higher. Bumped required CMake version
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[[Imported from SVN: r1853]]

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)