Make in Alpine: error in /mmproxy/libmill

Hi there,

I'm facing an issue when running `make` command in Alpine docker container :

- Log gist : https://gist.github.com/davidferlay/0adbcbe213ab0703e7ee94c85fb870e1
- Steps to reproduce : `docker run -it --privileged --sysctl net.ipv6.conf.all.disable_ipv6=0 alpine:3.11 ash`
```
apk --no-cache add git make autoconf automake libtool gcc iptables ip6tables g++
iptables -t mangle -I PREROUTING -m mark --mark 123 -j CONNMARK --save-mark
iptables -t mangle -I OUTPUT -m connmark --mark 123 -j CONNMARK --restore-mark
ip rule add fwmark 123 lookup 100
ip route add local 0.0.0.0/0 dev lo table 100
ip6tables -t mangle -I PREROUTING -m mark --mark 123 -j CONNMARK --save-mark
ip6tables -t mangle -I OUTPUT -m connmark --mark 123 -j CONNMARK --restore-mark
ip -6 rule add fwmark 123 lookup 100
ip -6 route add local ::/0 dev lo table 100
git clone https://github.com/cloudflare/mmproxy.git
cd mmproxy
git submodule update --init
make
```
I've seen example of mmproxy running in Ubuntu images ([here](https://github.com/andrewmichaelsmith/mmproxy/blob/master/Dockerfile) and [here](https://gitalpine.de/docker/mmproxy/-/blob/master/Dockerfile)) but I'd rather make it work on Alpine if possible

Any idea ?