I solved same a time ago. Personally, I'm using RTC to keeping correct (winter time) without DST. Together with RTC, I'm using the time library and there is DST based on check at each second. Time keeping var is 32bit and it is matter to compare this value with the DST var and then to set or not the time. I know that it sounds very complicated and you can afraid of the MCU would be too busy but I'm runnig this on ATmega1284P without any problem.