STK code has been updated for quite a while and nowadays it supports IPv6 quite fine. As such, the ebuild should be amended and its line 73 (-DUSE_IPV6=OFF # not supported by system enet) should be either changed to -DUSE_IPV6=ON or -DUSE_IPV6=$(usex ipv6) for more context, please check https://github.com/supertuxkart/stk-code/blob/73064a341afed6a63afd24651898bf0537f24374/CMakeLists.txt#L227
I don't think this is right, actually: it falls back to the bundled copy if USE_IPV6 is on.