can you add the same line for ubuntu ? The goal is to make Travis CI happy: it fails currently with
```
===== ERROR while installing hdf.0.9.1 =====
# opam-version 1.1.0
# os           linux
# command      ocaml setup.ml -configure --prefix /home/travis/.opam/system
# path         /home/travis/.opam/system/build/hdf.0.9.1
# compiler     system (4.01.0)
# exit-code    1
# env-file     /home/travis/.opam/system/build/hdf.0.9.1/hdf-8246-e4db2f.env
# stdout-file  /home/travis/.opam/system/build/hdf.0.9.1/hdf-8246-e4db2f.out
# stderr-file  /home/travis/.opam/system/build/hdf.0.9.1/hdf-8246-e4db2f.err
### stderr ###
# W: Field 'h4cc' is not set
# E: Cannot find external tool 'h4cc'
# E: Failure("1 configuration error")
```