iconv: remove usage on Linux in several packages

This fixes builds after #4419. Thanks to @vbgl for the original commit;
I changed that as I'm not sure whether passing null values to buildInputs is clean.

CC maintainers: @coroa, @peti, @phreedom, @robberer, @jcumming.

pkgs/applications/science/astronomy/stellarium/default.nix

@@ -1,4 +1,5 @@
-{ stdenv, fetchurl, cmake, freetype, libpng, mesa, gettext, openssl, qt4, perl, libiconv }:
+{ stdenv, fetchurl, cmake, freetype, libpng, mesa, gettext, openssl, qt4, perl
+, libiconvOrEmpty }:
 
 stdenv.mkDerivation rec {
   name = "stellarium-0.12.4";
@@ -8,7 +9,8 @@ stdenv.mkDerivation rec {
     sha256 = "11367hv9niyz9v47lf31vjsqkgc8da0vy2nhiyxgmk1i49p1pbhg";
   };
 
-  buildInputs = [ cmake freetype libpng mesa gettext openssl qt4 perl libiconv ];
+  buildInputs = [ cmake freetype libpng mesa gettext openssl qt4 perl ]
+    ++ libiconvOrEmpty;
 
   enableParallelBuilding = true;