[boost] icl and C++17

Hi, Boost icl doesn't compile using GCC 7.1 with -std=gnu++17. There's a pull request solving the issue[1], but icl seems non-maintained. It would be great if someone could upstream that change so that projects depending on icl can get upgraded to C++17. Thanks, Duarte [1] https://github.com/boostorg/icl/pull/9