Support epel-release on CentOS7

[akabe]

I need zeromq-devel, contained in epel-release, not the standard CentOS7 repository. I tried editing https://github.com/ocaml/opam-repository/blob/master/packages/conf-zmq/conf-zmq.0.1/opam like

opam-version: "1"
maintainer: "francois.berenger@inria.fr"
homepage: "http://zeromq.org/"
license: "LGPL"
build: [
  ["cc" "test.c" "-I/usr/local/include" "-L/usr/local/lib" "-lzmq"]
]
depexts: [
  [["debian"] ["libzmq3-dev"]]
  [["ubuntu"] ["libzmq3-dev"]]
  [["osx" "homebrew"] ["zeromq"]]
  [["alpine"] ["zeromq-dev"]]
  [["centos"] ["epel-release" "zeromq-devel"]]       # added epel-release
]

However, zeromq-devel cannot be installed because epel-release must be installed before zeromq-devel, i.e.,

$ yum install epel-release zeromq-devel

says that zeromq-devel is not found, while

$ yum install epel-release
$ yum install zeromq-devel

succeeds to install zeromq-devel.

I hope that depext supports such order-dependent installation. Some packages in https://github.com/ocaml/opam-repository require epel-release.