bump required CMake version down to 2.8.11 since no features are required in...

bump required CMake version down to 2.8.11 since no features are required in cmake>2.8.11 and this allows building on stock CentOS 7

CMakeLists.txt

-cmake_minimum_required(VERSION 2.8.12)
+cmake_minimum_required(VERSION 2.8.11)
 set_property(GLOBAL PROPERTY USE_FOLDERS ON)
 
 option(ENABLE_AMD_EXTENSIONS "Enables support of AMD-specific extensions" OFF)