[PATCH] [libcxx] Redo adding support for building and testing with an ABI library not along linker paths

[Chandler Carruth]

LGTM (I'm familiar with the CMake side of things, so happy to provide the review)

I think the real advantage of this is the improvement for an in-tree build. We could make find_library work for other cases even though its hard. But this is much nicer with the in-tree stuff.

http://reviews.llvm.org/D5860