musl headers sadly don’t conform to the preprocessor expression of the linux man
(_POSIX_C_SOURCE >= 200112L || _XOPEN_SOURCE >= 600) && ! _GNU_SOURCE
strerror_r, so there was no well defined preprocessor way of solving this.
In the end, the already very complex preprocessor define mix was dropped in favor of the usual dispatch by overloading a unifying wrapper [8e824ce].
You can grab the release from the releases page on GitHub.