Yup... welcome to the world of womanhood. Guys thinking that they have the privilege to eye you over like a piece of meat and undress you with their eyes is one of those great universal empowering/dehumanizing aspects of being a girl.
The first time it happened to me, I basically looked around to see if there was another woman behind me somewhere, because I was sure that it couldn't possibly be directed at me. But then when I saw the guy looking at me, I just sort of smiled, and then went off into a nervous corner somewhere because I was so sure that he was clocking me as trans and not whistling at me because I was attractive. (I had like no self-confidence at the time.)
Nowadays I just send the whistler a smirking half-grin, sort of a combination of smiling and rolling my eyes. And I guess that's my general reaction to it. I'm flattered, but also kind of horrified. I've never understood guys very well, not even in high school when this nonsense started in the first place, so my reaction is usually along the lines of "sigh... seriously, guys?"