This bug comes from the system wherever the package is built it must have some stale older version of libztd or something.
When built on a CLEAN debian-stable, the result is a binary that used the header of the libzstd version actually available in debian-stable → it doesn't have this problem. So. the problem is not the build-scripts, it's not the tor sources it's the system that used the build scripts. DO NOT ASK torproject to hide this. fix the issue. Which is as simple as use a plain debian stable to build, with the libzstd-dev from stable and it's fine. Thanks! --
