C: setenv implicit declaration

Question

For a school project we have to improve a bash-like prompt. Amongst other things, we have to implement the Linux's "cd" command. The only thing we are missing is to change the content of $OLDPWD (so we can use "cd -"). The thing is we keep getting a "implicit declaration of function 'setenv'" (because we need to use that function to update the value of said environment variable), even thought we have included stdlib.h, supposedly the only thing we need for it to work. As a side note, we also can't use unsetenv.

Has this happenned to someone else? Did we overlook something obvious? Thanks in advance.