Better defaults for in-tree libc++ with cmake.

This will detect if you are building libcxx in-tree and libcxxabi is
available.  If so, it will default to using the in-tree libcxxabi by
setting LIBCXX_CXX_ABI to "libcxxabi", LIBCXX_LIBCXXABI_INCLUDE_PATHS to
"${CMAKE_SOURCE_DIR}/projects/libcxxabi/include" and will add "cxxabi"
as a proper dependency.

Patch by Russell Harmon.