polkit: Don't build elogind on non-Linux.

In #72057 I added support for elogind when systemd is not supported,
but it occurred to me that it probably doens't make sense to have
either of systemd or elogind on Darwin.

pkgs/development/libraries/polkit/default.nix

@@ -55,7 +55,8 @@ stdenv.mkDerivation rec {
     ++ [ libxslt docbook_xsl docbook_xml_dtd_412 ]; # man pages
   buildInputs =
     [ expat pam spidermonkey_60 ]
-    ++ (if useSystemd then [systemd] else [elogind])
+    # On Linux, fall back to elogind when systemd support is off.
+    ++ stdenv.lib.optional stdenv.isLinux (if useSystemd then systemd else elogind)
     ++ stdenv.lib.optional withGnome gobject-introspection;
 
   propagatedBuildInputs = [