Fix the SFML renderer to work with non-MSVC compilers

In Renderers/SFML.cpp:

 m_Target.draw( &vert, 1, sf::PrimitiveType::Points );

should be:

 m_Target.draw( &vert, 1, sf::Points );

as Scoped Enums are part of C++11 / Microsoft Extensions and c++11 is not defined in the premake files, resulting in a "error: 'sf::PrimitiveType` is not a class or namespace" error with G++ 4.4.5.

I also had to add:
#include <cmath>

to the top of SFML.cpp because fabs was not declared.