Added conditional compilation for GLFW and SDL2
Added conditional compilation for GLFW and SDL2

Closes https://github.com/PistonDevelopers/piston/issues/396

placrosse/piston,KevinFrans3/piston,Potpourri/piston,bvssvni/piston,bvssvni/piston,tempbottle/piston,PistonDevelopers/piston,oldmanmike/piston,caldwell/piston,PistonDevelopers/piston,mitchmindtree/piston,getynge/piston,axelmagn/piston