It seems that libXi (at least version 1.3.1) requires tslib if built on a system that has it installed, but when built on one that doesn't have it, it doesn't need the library. Looks to me like a case of a missing USE flag. Found out because of indirectly broken smpeg due to a depclean that wasn't fixed by revdep-rebuild.
There's absolutely *no* reference whatsoever to tslib inside libXi. Unless you actually show us (something? anything?) how libXi was broken, I'm just going to assume something else was broken and you mistook it for libXi. Closing, thanks