Michael van der Westhuizen: The attached patch add support for building against libc++abi and libcxxrt to CMake builds of libc++.

Usage (with the appropriate CC and CXX environment variables) is:
$ cmake -DLIBCXX_CXX_ABI=libcxxabi '-DLIBCXX_LIBCXXABI_INCLUDE_PATHS=/home/michael/libcxxabi/include' ../libcxx
and:
$ cmake -DLIBCXX_CXX_ABI=libcxxrt '-DLIBCXX_LIBCXXRT_INCLUDE_PATHS=/home/michael/libcxxrt/src' ../libcxx

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@174623 91177308-0d34-0410-b5e6-96231b3b80d8