Well, I did a text sdearch in my arduino code home, and found the HTML file that describes "libiberty", wherethe function is described. I can also see that the object file is there, but there is no ".h" defining the prototype. Defining one myself works for compiling but fails on link. So - and this is where I gave up - one needs to specfically ask to include the lib/libiberty.a file in the link.
On a completly different note: The microcontroller chip has limited RAM. Doing things with malloc etc works, but people who do so usually very quickly exhaust memory. And being a microcontroller, there is no pretty "program memory exhausted" message as the program exits, as there is nothing to exit to; It just keeps on running, looping, crashing, hanging until the reset button is pressed.
Handle your own buffers.