memcached: disable treating warnings as errors on darwin

PIE hardening is not available on darwin but it's just a warning.

pkgs/servers/memcached/default.nix

···
 
   hardeningEnable = [ "pie" ];
 
+  NIX_CFLAGS_COMPILE = stdenv.lib.optionalString stdenv.isDarwin "-Wno-error";
+
   meta = with stdenv.lib; {
     description = "A distributed memory object caching system";
     repositories.git = https://github.com/memcached/memcached.git;