https://bugs.gentoo.org/982642#c9

Don't try to examine the config directory of the live system.
--- a/extra/generate_option_list.cc
+++ b/extra/generate_option_list.cc
@@ -83,7 +83,10 @@
     command << "\"";
   }
   command << " --verbose"
-             " --help";
+             " --help"
+             " --datadir=";
+  command << plugin_dir << "";
+
 #ifdef _WIN32
   command << "\"";
 #endif