Changes needed to get rust-rdkafka built

I've been trying to compile https://github.com/fede1024/rust-rdkafka with musl.

Its working with this docker image with the following changes:
1. install cmake:
```
apt-get install cmake
```
2. set the environment variable:
```
CXX=g++
```

I'd be happy to open a PR to support those changes here rather than needing to override it.