Note minimum required version of CMake

We already use features from 2.8.6, this just gives a slightly more friendly
message when the dependency isn't met.

Patch from Keith Walker.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175434 91177308-0d34-0410-b5e6-96231b3b80d8

CMakeLists.txt

@@ -1,7 +1,7 @@
 # See docs/CMake.html for instructions about how to build LLVM with CMake.
 
 project(LLVM)
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.6)
 
 # Add path for custom modules
 set(CMAKE_MODULE_PATH