Some features in Boost (e.g. Fiber library) require C++11 support. Tier1 and some Tier2 archs already come with C++11-capable toolchain in base system. Even those that don't are soon going to need it, anyway. So, let's take advantage.
This is going to break ABI, and some minor fallout is expected. Testing will be in bug 220714.
Here's the standalone patch: https://reviews.freebsd.org/D11582?vs=30742&id=30849
DragonFly folks sprinkled USE_CXXSTD=c++11 even on the ports that built fine e.g., audio/ardour or graphics/blender. I wonder, if it's due to https://gcc.gnu.org/wiki/Cxx11AbiCompatibility that may not affect libc++.
Landed as ports r450557.