Logic problem with alarm system?
As Oli noted, this does not require a microcontroller.  Your requirement is simple enough to do in dedicated hardware.

However, using a micro has some advantages: 

The light/dark thresholds can be easily adjusted if the LDRs are driven into A/D inputs and the decision done in firmware.  This also leaves more flexibility for changes as you get experience with the unit.  For example, you might want to detect sudden light changes, not just absolute level.

Lower power.  The dedicated circuit would be on all the time unless you make it complicated.  The micro can easily power down all the sensors and sleep most of the time.  Sampling the light levels every 500 ms is most likely plenty fast enough.  Even if it takes 100 &micro;s to turn on the LDRs, wait for things to settle, take the A/D readings, make the decision, and go back to sleep, that's still only 1/5000 of the time.  The rest of the time the total current draw should be 1 &micro;A or less.  If the current draw is 20 mA when on (that's high), the average would still be under 5 &micro;A.  Note that all these estimates have been quite pessimistic.

Less parts.  All you need is the micro, the on/off switch, the LDRs, a resistor per LDR, and a transistor to switch on the LDR circuit.