hashdeep: add darwin build failure note

pkgs/tools/security/hashdeep/default.nix

@@ -17,6 +17,9 @@ stdenv.mkDerivation rec {
     description = "A set of cross-platform tools to compute hashes";
     homepage = "https://github.com/jessek/hashdeep";
     license = licenses.gpl2;
+    # Build fails on Darwin:
+    # > ./xml.h:103:82: error: invalid suffix on literal; C++11 requires a space between literal and identifier [-Wreserved-user-defined-literal]
+    # >     void xmlout(const std::string &tag,const int64_t value){ xmlprintf(tag,"","%"PRId64,value); }
     platforms = with platforms; linux ++ freebsd ++ openbsd;
     maintainers = [ maintainers.karantan ];
   };