I recall there was some discussion in Gentoo at some point about handling C+11. While nothing specific was decided and most developers hold their packages to C98, there was a suggestion to introduce a new USE_EXPAND to handle C+ stdlib compatibility.

Something along the lines of:

# only one of:
CXX_STDLIB="gnu98 gnu11 libcxx"

I'm not sure if this will be revived but if so, it may conflict with Funtoo's use of USE=c++11 on packages such as icu and boost.