If my understanding is correct I see two possible ways forward. 1. Rebuild python3.8 against the new glibc. 2. Remove the stropts include from samba, it doesn't seem to actually be used for anything (at least I can't find any other references to HAVE_STROPTS_H in the code). I am currently testing a build in raspbian bullseye with the second approach. I will report back later on whether it results in a succesful build.
This build was successful and I uploaded it to raspbian, a debdiff should appear soon at https://debdiffs.raspbian.org/main/s/samba . No intent to NMU in Debian.