message: Increase the minimum CMake version to 2.8.12
  This is a followup to
  target_include_directories was introduced in 2.8.12, thus CMake 2.6 no
  longer works.
diff: project(pugixml)
  -cmake_minimum_required(VERSION 2.6)
  +cmake_minimum_required(VERSION 2.8.12)
  option(BUILD_SHARED_LIBS "Build shared instead of static library" OFF)
  option(BUILD_TESTS "Build tests" OFF)
  