The Cap'n Proto installation instructions say

>We will need to use libc++ (from LLVM) rather than libstdc++ (from GNU) because Xcode’s libstdc++ (like its GCC) is too old. In order for your freshly-downloaded Clang binaries to be able to find it, you’ll need to symlink it into the Clang tree
>`ln -s /usr/lib/c++ ~/clang-3.2/lib/c++`