Motivation for this change

With the upgrade to firefox 62 (and potentially before) clang flags are now required on i686.

This might just work as it is. I haven't had the time to run/check for rebuilds. I intend to do that in the evening / during the day.

It seems that this would also be required on stable and the 18.09 release branch.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Fits CONTRIBUTING.md.