/usr/include/SFML/System/ is in libsfml-dev 2.1+dfsg2-1+b2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/SFML/System/Clock.hpp
/usr/include/SFML/System/Err.hpp
/usr/include/SFML/System/Export.hpp
/usr/include/SFML/System/InputStream.hpp
/usr/include/SFML/System/Lock.hpp
/usr/include/SFML/System/Mutex.hpp
/usr/include/SFML/System/NonCopyable.hpp
/usr/include/SFML/System/Sleep.hpp
/usr/include/SFML/System/String.hpp
/usr/include/SFML/System/Thread.hpp
/usr/include/SFML/System/Thread.inl
/usr/include/SFML/System/ThreadLocal.hpp
/usr/include/SFML/System/ThreadLocalPtr.hpp
/usr/include/SFML/System/ThreadLocalPtr.inl
/usr/include/SFML/System/Time.hpp
/usr/include/SFML/System/Utf.hpp
/usr/include/SFML/System/Utf.inl
/usr/include/SFML/System/Vector2.hpp
/usr/include/SFML/System/Vector2.inl
/usr/include/SFML/System/Vector3.hpp
/usr/include/SFML/System/Vector3.inl