message: bump CMake required version for compiler checks
diff: set(CMAKE_DISABLE_SOURCE_CHANGES ON)
  set(CMAKE_DISABLE_IN_SOURCE_BUILD ON)
  -cmake_minimum_required(VERSION 2.8)
  +cmake_minimum_required(VERSION 3.1)
  cmake_policy(SET CMP0003 NEW) # since 2.6
  if(POLICY CMP0015)
  cmake_policy(SET CMP0015 NEW) # since 2.8.1
  