pam: fix cross-compilation from darwin

audit requires running linux-specific code during the build,
so it cannot be cross-compiled from darwin hosts. So we can only
enable audit support in pam when the buildPlatform is linux.

pkgs/os-specific/linux/pam/default.nix

@@ -20,7 +20,8 @@ stdenv.mkDerivation rec {
   nativeBuildInputs = [ flex ]
     ++ lib.optional stdenv.buildPlatform.isDarwin gettext;
 
-  buildInputs = [ cracklib db4 audit ]
+  buildInputs = [ cracklib db4 ]
+    ++ lib.optional stdenv.buildPlatform.isLinux audit
     ++ lib.optional withLibxcrypt libxcrypt;
 
   enableParallelBuilding = true;