changed macro MAX_G_FORCE to SDL_IPHONE_MAX_GFORCE and moved it to the SDL_config_iphoneos.h file. This should allow users to convert between the Sint16 returned by polling the joystick and units of g-force, which better describe what is going on with the iPhone (and are what the iPhone OS actually gives you). This conversion wouldn't be necessary except that we'd need floating point to store everything as g-force.