Now that we have a Centos 7.4 VM image, a handful of contributors have started a wider effort to get the master nix packaged for enterprise linux.

While that's a non-trivial effort and may even be fundamentally misguided given that the resulting RPM will have dependencies on other package distributions and a different ABI, in the meantime it turns out that the stable branch of nix builds on RHEL/CentOS 7.4 without any necessary modifications, aside from a minor backport of the RPM macro logic update to distinguish el6 from el7. Fortunately it was just a light touch, since eventually this will all be replaced with @abbradar's nice work cleaning up the RPM packaging.

This at least lets us get some coverage on Hydra build products for users interested in using nix in enterprise environments. I'm not sure if it's standard practice to add new releases to a maintenance branch (I see @grahamc added aarch64 last week, but I'd imagine that's atypical). Either way this PR at least shows users that it's not hard to build a nix RPM for el7.

Builds with nixpkgs from before the dblatex update last week; as it currently stands stable nix's documentation does not build with master nixpkgs until we upgrade dblatex.

$ nix-build release.nix -A rpm_centos74x86_64
...
[  251.473427] reboot: Power down
/nix/store/5vaa1pvbn0gy9pkqn00ff2f9wpj24ic3-nix-rpm-centos-7.4-x86_64-1.11.17pre1234_abcdef
$ tree result
result
├── nix-support
│   ├── full-name
│   └── hydra-build-products
└── rpms
    └── centos-7.4-x86_64
        ├── emacs-nix-1.11.17pre1234_abcdef-2.el7.centos.noarch.rpm
        ├── emacs-nix-el-1.11.17pre1234_abcdef-2.el7.centos.noarch.rpm
        ├── nix-1.11.17pre1234_abcdef-2.el7.centos.src.rpm
        ├── nix-1.11.17pre1234_abcdef-2.el7.centos.x86_64.rpm
        ├── nix-debuginfo-1.11.17pre1234_abcdef-2.el7.centos.x86_64.rpm
        ├── nix-devel-1.11.17pre1234_abcdef-2.el7.centos.x86_64.rpm
        └── nix-doc-1.11.17pre1234_abcdef-2.el7.centos.noarch.rpm

3 directories, 9 files