Does this not work if you add it to the APPLE section in your CMakeLists.txt?
find_library(libiconv NAMES libiconv)
find_library(libz NAMES libz)
I’m not sure if you need to pass specific paths to the find_library, but if you do then it should be just a matter of finding out where those libraries are usually placed.
EDIT: Looks like this has been reported here, with suggestions on how to get it working as well.