The PNA4603H (available from Digikey) outputs a voltage that is proportional to the light level. Its sensitivity and wavelength are similar to that of the human eye.

Use it on an ADC pin or use it on one input to the comparator and use a potentiometer on the other input to the comparator.

If you supply it with +5V using a PIC pin and use ADC, you can turn it off between readings to minimize battery drain.