I've grepped thoroughly through lcms sources and despite --with-zlib option to configure lcms doesn't use zlib directly. Zlib can be used indirectly via media-libs/tiff if tiff use flag set, but I don't see why lcms needs zlib use flag.
Thanks, this was fixed in git commit 0d0592550d27c8d666bc9e8abf22c84f49ec9386