glibc: Fix building with musl on GCC 8.

GCC 8 introduced new warnings that were picked up by -Werror;
this commit makes them non-errors until fixed upstream.

See

* https://github.com/NixOS/nixpkgs/pull/68244#issuecomment-544307798
* https://github.com/nh2/static-haskell-nix/issues/56

This commit takes care to not change the derivation for non-musl builds.