switch(global.primary)
{
    case "left":
        if(mouse_check_button(mb_left))
        {
            //dowhatever
        }
        break;
    case "middle":
        if(mouse_check_button(mb_middle))
        {
            //dowhatever
        }
    case "right":
        if(mouse_check_button(mb_right))
        {
            //dowhatever
        }
}