Fix/Improve Build on Linux

This fixes several issues with building rbSFML.

* The `-fPIC` flag is readded, since it is needed on Linux
* The `include` and `lib` checks in the Rakefile are removed, since these files aren't required if SFML was properly installed on Linux
* The tests are changed to `require` the latest build, rather than the installed build. This lets you test rbSFML without installing it.