Without this, the C++ dynamic_cast<> operator does not work reliably in conjunction with dynamic linking. See http://www.russellmcc.com/posts/2013-08-03-rtti.html (That article suggests that Apple builds libc++abi without _LIBCXX_DYNAMIC_FALLBACK, but "strings /usr/lib/libc++abi.dylib" shows that it *is* enabled.)