You could use a comparator which will give a pretty sharp 'on/off' threshold. With hysteresis to stop is oscillating states at the threshold. The LM393 as far as I recall, uses very little power and depending on the resistor values and battery voltage - its quiescent current draw should be quite low.
A possible alternative to an LDR would be a photodiode, or perhaps even an LED as a light sensor. Both can produce small votlages based on the incident light, at very low currents - enough to trip a comparator, but low enough to use uA when in the 'off' state.