Cleanup of nix.spec file to allow build on EL7 and beyond
Cleanup of nix.spec file to allow build on EL7 and beyond

- Clean deprecated spec file macros ( < EL6 )
- Allow to build Nix on EL7 using Software collection and Copr backports
- Add conditional build to build Nix RPM without documentation for dev builds
- Add missing BuildRequires (openssl, and others )

ehmry/nix,NixOS/nix,ehmry/nix,layus/nix,layus/nix,ehmry/nix,rycee/nix,dtzWill/nix,layus/nix,dtzWill/nix,NixOS/nix,zimbatm/nix,pikajude/nix,vcunat/nix,rycee/nix,zimbatm/nix,zimbatm/nix,peti/nix,peti/nix,NixOS/nix,NixOS/nix,pikajude/nix,peti/nix,pikajude/nix,ehmry/nix,zimbatm/nix,layus/nix,rycee/nix,vcunat/nix,dtzWill/nix,shlevy/nix,ehmry/nix,dtzWill/nix,ehmry/nix,rycee/nix,NixOS/nix,shlevy/nix,vcunat/nix,peti/nix,vcunat/nix,shlevy/nix,peti/nix,shlevy/nix,layus/nix,shlevy/nix,dtzWill/nix,rycee/nix