r-modules/generic-builder: Add missing iconv library to fix build on darwin

[tricktron]

Motivation for this change

Executing nix-shell --packages 'rWrapper.override{packages = [ rPackages.SparseM ];}' fails with the error:

ld: library not found for -liconv
clang-7: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [/nix/store/saj094y98yryxgi7q1ja36hkbq52yvkf-R-3.6.2/lib/R/share/make/shlib.mk:6: SparseM.so] Error 1
ERROR: compilation failed for package 'SparseM'

Adding the missing iconv library for darwin fixes the issue.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• 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 nixpkgs-review --run "nixpkgs-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)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[peti]

Please don't add that package in the generic builder. Add it to SparseM, which actually needs it.

[tricktron]

@peti Thanks for your quick reply. Sorry for the dumb sounding question but how can I add the missing library to the SparseM package? It is a cran package and thus automatically generated with the generate-r-packages.R script.

[peti]

See https://github.com/NixOS/nixpkgs/blob/master/pkgs/development/r-modules/default.nix#L267 for an example.

[tricktron]

@peti Thanks for the hint. I reverted the changes in generic-builder.nix and added the libconv library to the r-packages that I used and were breaking directly. In addition, I did a refactoring in a second commit (with statement scope & sorting using sort -t '=' -k 2).

[peti]

Please don't make sweeping changes to that file -- especially, if those changes are unrelated to the actual topic of the PR.

[tricktron]

@peti Ok, I reverted the refactoring changes.