Great news! My faith in humanity and the victory of sanity has been restored.
I've now pushed on mariadb-10.5 master the necessary changes in https://salsa.debian.org/mariadb-team/mariadb-10.5/-/commit/ca2574aa88434d1c49456c677b7dcb904902daaf I will keep this issue open, and start excluding extras/wolfssl in the gbp.conf next time I import a new upstream (instead of the d/rules hack done now).
