ROOT won't build on clang (libc++) 4

libc++ recently made a deep change to string_view, as described in http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/p0254r1.pdf.

After that, to_string() stopped being a valid method on string_view, which I think gets everything into a weird state. From what I can tell, the CMake build checks for to_string() on string_view and then decides to bring in experimental_string_view, which breaks in a similar way.