karlok:
Can I supply 5V to the 4.5V laser pointer without problems? I tested it with one I have at home, and it worked, will it work again, for longer times?
It depends upon the laser unit in question. Some of them can tolerate the additional voltage without noticable damage or shortened working lifetime, others will have problems in the long run. The only way to be sure is find-out the ratings for the specific laser you are using.
karlok:
I read that a reference value for the photoresistor's value should be taken (reference: with normal light (either day or night); and the one the laser lights on). These values can be compared, but how should I compare them? What about: if v1~v2 then alarm, that means if they have circa the same value. I could say "abs(v1-v2 )< 50 then alarm", what about this ? Calibration wouldn't work because the room I am speaking about changes its brightness during the day very much.
A laser shining directly onto the photoresistor should be significantly brighter than even full daylight and/or normal room illumination. Therefore the threshold will be a much greater voltage than the possible with ambient light no matter what time of day or the amount of ambient lighting.