The logic behind 'libiconvOrLibc' is flawed and that attribute should be removed.

See https://github.com/NixOS/nixpkgs/pull/2532 for further details.

pkgs/top-level/all-packages.nix

@@ -5085,6 +5085,9 @@ let
     then null
     else libiconv;
 
+  # The logic behind this attribute is broken: libiconvOrNull==null does
+  # NOT imply libiconv=glibc! On Darwin, for example, we have a native
+  # libiconv library which is not glibc.
   libiconvOrLibc = if libiconvOrNull == null then gcc.libc else libiconv;
 
   # On non-GNU systems we need GNU Gettext for libintl.