Cannot build statically linked cargo on musl with RUSTFLAGS="-C target-feature=+crt-static"

```
$ RUSTFLAGS="-C target-feature=+crt-static" cargo build --release
error: cannot produce proc-macro for `serde_derive v1.0.36` as the target `x86_64-alpine-linux-musl` does not support these crate types
```

It does not work since cargo 0.18.0, there were no problem with static build in 0.17.0.