libunwind depends on C++ library headers. When building libunwind as part of LLVM and libc++ is available, use its headers.
I would name this variable with _DEFAULT on the end so that it is clear that you're setting the default value for a cached variable.
Pretty sure include_directories can't be called with an empty string, so in the case where find_path fails, this could go wrong.
LGTM! Thanks!
I would name this variable with _DEFAULT on the end so that it is clear that you're setting the default value for a cached variable.