/usr/include/SFML/System/ is in libsfml-dev 1.6+dfsg1-2ubuntu2.
This file is owned by root:root, with mode 0o755.
..
/usr/include/SFML/System/Clock.hpp
/usr/include/SFML/System/Lock.hpp
/usr/include/SFML/System/Mutex.hpp
/usr/include/SFML/System/NonCopyable.hpp
/usr/include/SFML/System/Randomizer.hpp
/usr/include/SFML/System/Resource.hpp
/usr/include/SFML/System/Resource.inl
/usr/include/SFML/System/ResourcePtr.inl
/usr/include/SFML/System/Sleep.hpp
/usr/include/SFML/System/Thread.hpp
/usr/include/SFML/System/Unicode.hpp
/usr/include/SFML/System/Unicode.inl
/usr/include/SFML/System/Unix/
/usr/include/SFML/System/Unix/Mutex.hpp
/usr/include/SFML/System/Unix/Thread.hpp
/usr/include/SFML/System/Vector2.hpp
/usr/include/SFML/System/Vector2.inl
/usr/include/SFML/System/Vector3.hpp
/usr/include/SFML/System/Vector3.inl
/usr/include/SFML/System/Win32/
/usr/include/SFML/System/Win32/Mutex.hpp
/usr/include/SFML/System/Win32/Thread.hpp