fix debug.cpp error output (now compiles against libc++)

src/debug.cpp

@@ -137,7 +137,7 @@ void logger::use_file(std::string const& filepath)
         else
         {
             std::stringstream s;
-            s << "cannot redirect log to file " << file_output_;
+            s << "cannot redirect log to file " << file_name_;
             throw std::runtime_error(s.str());
         }
     }