[PATCH] Fix re-building in-tree libc++ against in-tree libc++abi

[Viktor Kutuzov]

Also, it seems we should replace 'NOT DEFINED LIBCXX_CXX_ABI' with just 'NOT LIBCXX_CXX_ABI' as LIBCXX_CXX_ABI is defined during successive builds.

http://reviews.llvm.org/D4805