fetchurl: cleanup, better errors

Also fix what seems like bugs in uncommon `stdenv`s.

pkgs/stdenv/darwin/default.nix

@@ -118,6 +118,7 @@ in rec {
         initialPath  = [ bootstrapTools ];
 
         fetchurlBoot = import ../../build-support/fetchurl {
+          inherit lib;
           stdenvNoCC = stage0.stdenv;
           curl = bootstrapTools;
         };

pkgs/stdenv/freebsd/default.nix

@@ -29,7 +29,8 @@ let inherit (localSystem) system; in
     inherit bootstrapTools;
 
     fetchurl = import ../../build-support/fetchurl {
-      inherit stdenv;
+      inherit lib;
+      stdenvNoCC = stdenv;
       curl = bootstrapTools;
     };
 

pkgs/stdenv/native/default.nix

@@ -131,7 +131,7 @@ in
     };
 
     fetchurl = import ../../build-support/fetchurl {
-      inherit stdenv;
+      inherit lib stdenvNoCC;
       # Curl should be in /usr/bin or so.
       curl = null;
     };