libvdpau: break evaluation cycle on darwin

That path isn't meaningful on non-NixOS anyway.

pkgs/development/libraries/libvdpau/default.nix

···
 
   propagatedBuildInputs = [ xorg.libX11 ];
 
-  configureFlags = [ "--with-module-dir=${mesa_noglu.driverLink}/lib/vdpau" ];
+  configureFlags = stdenv.lib.optional stdenv.isLinux
+    "--with-module-dir=${mesa_noglu.driverLink}/lib/vdpau";
 
   installFlags = [ "moduledir=$(out)/lib/vdpau" ];