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/development/ocaml-modules/ocaml-text/default.nix

@@ -1,4 +1,4 @@
-{stdenv, fetchurl, libiconv, ocaml, findlib, ncurses}:
+{stdenv, fetchurl, libiconvOrNull, ocaml, findlib, ncurses}:
 
 stdenv.mkDerivation {
   name = "ocaml-text-0.6";
@@ -8,9 +8,12 @@ stdenv.mkDerivation {
     sha256 = "0j8gaak0ajnlmn8knvfygqwwzs7awjv5rfn5cbj6qxqbxhjd5m6g";
   };
 
-  buildInputs = [ocaml findlib libiconv ncurses];
+  buildInputs = [ocaml findlib ncurses]
+    ++ stdenv.lib.optional (libiconvOrNull != null) libiconvOrNull;
 
-  configurePhase = "iconv_prefix=${libiconv} ocaml setup.ml -configure";
+  configurePhase =
+    (stdenv.lib.optionalString (libiconvOrNull != null) "iconv_prefix=${libiconvOrNull} ")
+    + "ocaml setup.ml -configure";
 
   createFindlibDestdir = true;
 
@@ -18,9 +21,7 @@ stdenv.mkDerivation {
   meta = {
     homepage = "http://ocaml-text.forge.ocamlcore.org/";
     description = "OCaml-Text is a library for dealing with ``text'', i.e. sequence of unicode characters, in a convenient way. ";
-    license = "BSD";
+    license = stdenv.lib.licenses.bsd3;
     platforms = ocaml.meta.platforms;
-    maintainers = [
-    ];
   };
 }