Panting is just a bird's way of keeping cool, just like dogs pant. They may look pretty miserable, and I'm sure they hate the heat, but the panting helps them evaporate water in their bodies, which cools them down internally. And always make sure they have cool, clean water!
Also look out for a chicken who may be suffering from sun stroke or heat stress- they may pant harder than normal, or be abnormally listless and inactive. Then, as soon as you notice symptoms like that, try and cool them down by taking them inside, misting them off, even placing ice cubes or a cool, damp towel under them.