fatsort: fix build for cross FreeBSD

libiconv is already defined per-platform.  The actual libiconv library
won't be built on platforms like Linux where it doesn't need to be, so
there's no need to maintain a separate platform list here.

We have to override the uname values, or they'll be detected for the
build platform, because the build system just runs uname(1).

Tested by cross compiling to x86_64-freebsd from x86_64-linux.

pkgs/tools/filesystems/fatsort/default.nix

@@ -14,12 +14,14 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-mm+JoGQLt4LYL/I6eAyfCuw9++RoLAqO2hV+CBBkLq0=";
   };
 
-  buildInputs = [ help2man ]
-    ++ lib.optionals stdenv.isDarwin [ libiconv ];
+  buildInputs = [ help2man libiconv ];
 
   makeFlags = [
     "CC=${stdenv.cc.targetPrefix}cc"
     "LD=${stdenv.cc.targetPrefix}cc"
+
+    "UNAME_O=${stdenv.hostPlatform.uname.system}"
+    "UNAME_S=${stdenv.hostPlatform.uname.system}"
   ];
 
   # make install target is broken (DESTDIR usage is insane)