set rl_change_environment in readline gt v6.3

Looked up GNU readline library's repo and looked
for the first occurance of rl_change_environment.

src/nnn.c

@@ -4783,7 +4783,9 @@ int main(int argc, char *argv[])
 	setlocale(LC_ALL, "");
 
 #ifndef NORL
+#if RL_READLINE_VERSION >= 0x0603
 	rl_change_environment = 0;
+#endif
 	/* Bind TAB to cycling */
 	rl_variable_bind("completion-ignore-case", "on");
 #ifdef __linux__