Update cmake minimum version to 2.8.12

CMakeLists.txt

@@ -3,7 +3,7 @@ if(${CMAKE_SOURCE_DIR} STREQUAL ${CMAKE_BINARY_DIR})
 	message(FATAL_ERROR "Prevented in-tree build.")
 endif()
 
-cmake_minimum_required(VERSION 2.8)
+cmake_minimum_required(VERSION 2.8.12)
 
 include(ExternalProject)