add SYSTEM to add_subdirectory calls for bundled libraries

[cz4rs]

CMake 3.25 introduces SYSTEM argument to add_subdirectory. Use that for all our bundled libraries to have them included with -isystem rather than -I.

https://cmake.org/cmake/help/latest/command/add_subdirectory.html

Tagging this with c++20 because we currently have cmake_minimum_required 3.23. We should be able to bump CMake requirements along with adopting new standard.