D_GNU_SOURCE option for gcc?

["Thomas H. George" <xyz@spininternet.com>]

I am using source code from UNIX SYSTEMS PROGRAMMING by Robbins & 
Robbins.  It gives a compiler error unless compiled with the option 
-D_GNU_SOURCE .
Where does this option come from - its not in the man pages?  Why is it 
needed when using the Debian gcc and g++ packages?

Curious,

Tom