To answer my own question and help others long the way, the issue is that c++ has moved on and urbackup is somewhat stuck in its time. The std::auto_ptr function has been removed from clang/c++ versions >=17. To reenable, you must configure as follows:
./configure --enable-embedded-cryptopp --enable-embedded-zstd CXXFLAGS=-D_LIBCPP_ENABLE_CXX17_REMOVED_AUTO_PTR
The better solution would be to replace std:auto_ptr with std:unique_ptr, but I did not want to go that route because this has not been tested, and I would be worried about the integrity of my backups. The replacement would affect many source files. Good luck!