Special case for the epel-release yum-package

[smondet]

Fixes #70, (cf. also #76).

Tested with hdf5:

 $ docker run -it  ocaml/opam:centos-7_ocaml-4.03.0 bash
 $ opam pin add depext 'https://github.com/smondet/opam-depext.git#epel-release-special-case'
...
 $ opam depext --version
1.0.5+dev
 $ opam depext --yes hdf5
...
 $ opam installl --yes hdf5
...

[avsm]

I don't mind special-casing epel, but isn't this patch doing so unconditionally? I was thinking that something like centos:epel as an additional tag might be appropriate, so that we can search for it in the list for the package and add the epel repository on demand.

[UnixJunkie]

nice hack

[smondet]

@avsm I did it like this because that's what seems to be already in use the
opam-repo. People specify the epel-release package when they need it (which
usually means that the depext error msg is good enough for CentOS interactive
users to fix it), like:

 [["centos"] ["epel-release" "hdf5-devel"]]

Cf. https://github.com/ocaml/opam-repository/search?utf8=%E2%9C%93&q=epel-release&type=

To pass CI builds, there is this “run some script from the internet” solution also:
ocaml/opam-repository@24ae1fb

  [["source" "centos"] ["https://gist.githubusercontent.com/akabe/9352e05d2413ccbefb2b52ea6ae81599/raw/7ad60a2be120849341cd7c5bf14656773cb7d405/centos-zmq.sh"]]

How do you think the depexp entry should look like?

 [["centos" "epel"] ["hdf5-devel"]]

or

 [["centos:epel"] ["hdf5-devel"]]

[UnixJunkie]

I personaly like the centos:epel one

[smondet]

@avsm any idea to move forward with this?
If you prefer any other definition than [["centos"] ["epel-release" "hdf5-devel"]] (the current one in the opam-repo), just say so :)

[AltGr]

I am not sure what to do with this with the current redifinition of depexts: ocaml/opam#3074 , and especially the replacement for the "tags" here: ocaml/opam#3058.

The patch as is would still actually work still, and may be the simplest solution!

[avsm]

This is ok by me; it just prioritises the epel package installation. A note in the README about how to specify the use of epel in centos depexts would be good. @AltGr says he'll merge this in the latest release.