log4shib: fixup build with gcc 11

https://hydra.nixos.org/build/173673855

pkgs/development/libraries/log4shib/default.nix

@@ -12,6 +12,8 @@ stdenv.mkDerivation {
 
   nativeBuildInputs = [ autoreconfHook ];
 
+  NIX_CFLAGS_COMPILE = [ "-std=c++11" ];
+
   meta = with lib; {
     description = "A forked version of log4cpp that has been created for the Shibboleth project";
     maintainers = [ maintainers.jammerful ];