Compiler warnings on older distributions

On Ubuntu `jauntry` 9.04 the noticed the following warning, probably from #298.

```
  CC       utils.o
utils.c: In function &lsquo;_our_safe_strndup&rsquo;:
utils.c:104: warning: implicit declaration of function &lsquo;strndup&rsquo;
utils.c:104: warning: incompatible implicit declaration of built-in function &lsquo;strndup&rsquo;
```