problem with g++ 2.9

[a]

in my c++ program i write:  
XtAddCallback(entry, XtNcallback, print_string, "one");

g++ 2.9 complains:
xzip.cpp: In function `int main(int, char **)':
xzip.cpp:35: passing `const char *' as argument 4 of
`XtAddCallback(_WidgetRec *, const char *,
void (*)(_WidgetRec *, void *, void *), void *)' discards qualifiers

earlier g++ does not complain