dprintf() is in POSIX.1-2008 (http://pubs.opengroup.org/onlinepubs/9699919799/functions/dprintf.html). So it should be enough if _POSIX_C_SOURCE was defined to 200809L. But with uClibc I need to define _GNU_SOURCE for the function to be declared in stdio.
The same is true for getline().
before you go on, yes, this is a TODO: "Go through SUSv4" http://git.busybox.net/uClibc/tree/TODO#n42