You need to tell clang you're using libc++ (the implementation of the standard library that supports C++11). The default implementation of the standard library doesn't support C++11, so SFML was built with libc++. By not specifying libc++ in your building options, clang is using the other implementation, so you get conflicts that result in weird errors (because SFML wants one, but you're using the other).
Add -stdlib=libc++ to you call to clang++