Why gcc gives warning: implicit declaration of function qsort_r?

I do include<stdlib.h> where qsort_r is given. And I use gcc -std=c99 -O3 myfun.c -o myfun to compile.

It compiles, links and runs well. I don't know why I got this warning and what is potential risk of this warning ?

BTW, my compiler is gcc version 4.7.3 (Ubuntu/Linaro 4.7.3-1ubuntu1)