How can I compile code that uses getsubopt()?

Question

I want to parse a list of options of the form key1=val1, key2=val2, etc (like the options to mount -o). The getsubopt() function seems perfect for this task (http://www.gnu.org/s/hello/manual/libc/Suboptions.html). However, when I try to compile my code using gcc, I get:

warning: implicit declaration of function ‘getsubopt’

and the program segfaults when I run it.