buildRustCrateTests: Fix link order test on darwin

As it turns out Darwin does most of the things differently then "normal"
systems. They are using a different shared library extension and require
an obscure commandline parameter that has to be added to every build
system out there. That issue seems to be with clang on Darwin as on
Linux that flag isn't required to build the very same tests (when using
clang).

After adjusting these two details the tests are running fine on the
darwin box that I was able to obtain.

pkgs/build-support/rust/build-rust-crate/test/default.nix

@@ -1,4 +1,13 @@
-{ lib, buildRustCrate, runCommand, runCommandCC, writeTextFile, symlinkJoin, callPackage, releaseTools }:
+{ lib
+, stdenv
+, buildRustCrate
+, runCommand
+, runCommandCC
+, writeTextFile
+, symlinkJoin
+, callPackage
+, releaseTools
+}:
 let
   mkCrate = args: let
     p = {
@@ -284,12 +293,18 @@ let
           ];
         };
         buildInputs = let
-          compile = name: text: runCommandCC name {} ''
-            mkdir -p $out/lib
-            $CC -shared -o $out/lib/${name}.so ${writeTextFile {
+          compile = name: text: let
+            src = writeTextFile {
               name = "${name}-src.c";
               inherit text;
-            }}
+            };
+          in runCommandCC name {} ''
+            mkdir -p $out/lib
+            # Note: On darwin (which defaults to clang) we have to add
+            # `-undefined dynamic_lookup` as otherwise the compilation fails.
+            cc -shared \
+              ${lib.optionalString stdenv.isDarwin "-undefined dynamic_lookup"} \
+              -o $out/lib/${name}${stdenv.hostPlatform.extensions.sharedLibrary} ${src}
           '';
           b = compile "libb" ''
             #include <stdio.h>