Disable extensions and force standard

CMakeLists.txt

@@ -4,6 +4,8 @@ if (CMAKE_VERSION VERSION_LESS "3.1")
   add_definitions(-std=c++11)
 else()
   set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_CXX_EXTENSIONS OFF)
 endif()
 
 if (POLICY CMP0048)