libc++ builds

Does the new libc++ library have to be built independent of llvm/clang?
It would be nice if one could just drop its source tree into the llvm tree
ala clang and do a single build for all three together.
ps Can libc++ be used in the llvm-gcc-4.2 build? Can one use the approach

rm -fr ../llvm-gcc-4.2-4.2.source/libstdc++-v3

and passing configure --with-gxx-include to build llvm-gcc-4.2 against
the installed libc++?

Currently libc++ depends upon a low-level library called libc++abi that is not part of llvm. libc++abi is essentially gcc's libsupc++ (not exactly, but very close). I believe the current lack of this low-level layer within llvm would complicate dropping it in as you (rightly) desire.