spurious use of %m format in pg_upgrade

pg_upgrade's pg_scandir_internal() makes use of the non-standard %m
format:
   pg_log(PG_FATAL, "could not open directory \"%s\": %m\n", dirname);

Is this an oversight, or is there an undocumented assumption that this
code will only be used on platforms where %m works?

(Which platforms don't have scandir() anyway?)