Make sure you really use a recent version of SFML 2. In case you want to compile the newest Thor (which I recommend), download also the newest Git revision of SFML. Then, build and
install it, see also the
installation tutorial on my website. The installation leads to the creation of include and lib directories inside the specified CMAKE_INSTALL_PREFIX.
After installing SFML, begin to build Thor, and set the SFML_ROOT CMake variable to the just mentioned SFML install path.
By the way, I haven't tested clang, so I would be very glad about some feedback. But if
this list is correct, at least clang 3.1 is required for lambda expressions, which are used by Thor 2.0.