I am at fltk 1.3.5 but also trying 1.3.8

Doc says I can provide my own versions of Fl::error() and Fl::fatal() to override the default actions (basically exit).  I am providing this code in my routine, compiling with gcc, and I get an error message "no 'void Fl::error(const char*, ...)' member function declared in class 'Fl'.

My code is this:

void Fl::error(const char *fmt, ...) __attribute__((noreturn));

void Fl::error(const char *fmt, ...) { <some code> }

What do I have to do to replace the default functions with my functions?