I don't understand what a potential solution would be. The various projects use -std=c++XXX because that's what their code requires. -std=c++XXX can't generally be changed. If a dependent project is incompatible that's no different than any other case of incompatible dependencies in Gentoo.
I think -std=c++XXX discussions before happened because gcc changed the C++ ABI with -std=c++11. I don't think that's particularly relevant here, since as far as I know different -std=c++XXX values don't change the ABI with current gcc. So I guess my understanding is that there isn't a problem different than existing incompatible dependencies, but maybe I have misunderstood you.
