add an ugly missing #ifdef USE_GZIP to allow compilation without the flag set

Mordante: it's just a quick fix to avoid annoying complains.

src/gamestatus.cpp

@@ -736,9 +736,12 @@ void read_save_file(const std::string& name, config& cfg, std::string* error_log
 
 	cfg.clear();
 	try{
+		#ifdef USE_GZIP
 		if(is_gzip_file(name)) {
 			read_gz(cfg, *file_stream, error_log);
-		} else {
+		} else
+		#endif
+		{
 			detect_format_and_read(cfg, *file_stream, error_log);
 		}
 	} catch (config::error &err)