Well, t the risk of (not so) steal boasting, I'm on he receiving end of pretty regular whistles - although they tend to be white van men rather than builders - and flattering it is not. 
Why would it be? Sorry mate, I can do better. Way better.
I don't get why you'd be flattered by the the attention of someone with no discernment whatsoever, and who would probably hump a lamp-post if it would put out.
I mean, you don't see the likes of James Bond wolf-whistling at women, do you?!
Men that can actually get women don't really need to wolf whistle, is all I'm saying...
I'm going to be honest and admit I cringe a bit inside when a women admits to finding such crumbs flattering. :-/
As an aside, I passed a young girl, about 14, walking past a group of 3 lads about the same age as her recently, doing the whole phwoar, loaded comments bit and the fear was practically emanating off her. :( It was awful. It was daylight and there were people around, but I waited until she had got to a safe point before walking on. I was shaken just from witnessing it...