@Cubbi: what is libc++ then? Compared to libstdc++?
libstdc++ is the GNU C++ standard library that accompanies the GNU g++ compiler. It relies on GNU C library for the shared features and supports whatever platforms g++ supports. It's missing a lot of minor and several major C++11 requirements. It doesn't directly support other compilers, but because it is ubiquitious, other compilers implement whatever g++ quirks are needed to be used with it (clang++ and intel did that, for example)
libc++ is a complete (I think, the only complete so far) C++11 standard library implementation written from scratch for Apple. It supports MacOS and relies on BSD C library, although there is work ongoing to support Linux and Windows (it mostly works, but some features are missing). It expects clang++ as the compiler.
llvm is good in general. It can be used for on-the-fly or JIT compilation, it can be used as a backend and optimizer to several different platforms for a compiler or VM. It's just... I love it. I would probably marry it if it were legal in my state.
Back on topic, libc++ is co-developed by Apple because Clang is big on Mac OSX. I don't think they created it although don't quote me. It still uses extensions heavily for optimization and it's main goal was licensing differences. libstdc++ works perfectly fine with Clang (as I use it interchangeably with gcc) and supposedly, vice versa (I don't have a Mac nor do I care to test on Linux).