Fixes CMakeLists.txt library for os x (and possibly linux).

`(SFML/BOOST)_ROOT` variables were being used regardless of whether or not `find_package` successfully located a library, which is silly for nix systems. Modified CMakeLists.txt to only use specified "root" variables if and only if `find_package` doesn't find the library.

Someone on Windows or something verify that this works for you as well.