The main section of this patch teaches CMake a new option LIBCXXABI_LIBCXX_PATH that specifies the path to the libcxx source root. This information is passed to lit so that it can better find libc++'s python module. LIBCXXABI_LIBCXX_PATH is also used to help find the libc++ headers.
The rest of this patch is misc cleanup, mostly to make pep8 and pylint happy.
I've also copied libc++'s .gitignore into libc++abi.