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.
Changes requested by LB

Uses directory found by find_package if and only if *_ROOT is not specified.
CMakeLists.txt fixes for windows (again)
Third time's the charm.
Merge pull request #68 from Thumperrr/master

Fixes CMakeLists.txt library for os x (and possibly linux).

Merged because the code built for me, Thumper, and Naraku (according to Thumper)
Fix use of invalidated iterator