Issue title: Missing beta1 depext on alpine
 Issue body: **Describe the bug**
When building beta1 on alpine, depext is missing `automake` and `autoconf`.

**To Reproduce**
```dockerfile
from ocaml/opam2:alpine-3.9 as builder

copy deps deps
run sed -i '/#/d' deps

run wget http://downloads.sourceforge.net/liblo/liblo-0.30.tar.gz
run tar xf liblo*
run cd liblo-0.30 && ./configure && make && sudo make install

run opam pin -n liquidsoap https://github.com/savonet/liquidsoap.git#1.4.0-beta1
run opam depext $(cat deps)
run sudo apk add --no-cache automake autoconf      # <------- should not be needed
run opam install --destdir=/home/opam/ls $(cat deps)
run opam depext -ln $(cat deps) > /home/opam/depexts
```

 Comments: 
 Comment 0: Nice! 
{'source': 'GithubIssues', 'repo_name_with_owner': 'savonet/liquidsoap', 'issue_id': 451070866, 'repo_stars': 1024.0}