Introduce libiconvOrNull and libiconvOrGlibc

One can use these attrs instead of custom 'if ... then libiconv else null'.
This way we can maintain the correct 'if ... then' check in one place.

NB: I don't know what is the correct condition, so I use stdenv.isLinux. Feel
free to fix this.

svn path=/nixpkgs/trunk/; revision=31906

pkgs/top-level/all-packages.nix

@@ -4054,6 +4054,10 @@ let
 
   libiconv = callPackage ../development/libraries/libiconv { };
 
+  libiconvOrNull = if gcc ? libc then null else libiconv;
+
+  libiconvOrLibc = if gcc ? libc then gcc.libc else libiconv;
+
   libid3tag = callPackage ../development/libraries/libid3tag { };
 
   libidn = callPackage ../development/libraries/libidn { };