dirfd(3) is now specified in POSIX.1-2008, so it should be available
when the _POSIX_C_SOURCE macro is defined to >= 200809L

However, /usr/include/dirent.h only exposes the dirfd() macro if
__BSD_VISIBLE is defined.  However, __BSD_VISIBLE appears to be false
if _POSIX_C_SOURCE is defined.

Fix: 

It's probably best to add dirfd() as a function to libc, allowing the opacity
of the struct _dir_desc to be preserved.
How-To-Repeat: /*
 * To build + link:
 *   gcc -o dirfdtest dirfdtest.c
 * having _POSIX_C_SOURCE defined will cause this to fail link
 */
#define _POSIX_C_SOURCE 200809L
#include <dirent.h>
int main(void)
{
        DIR *dir = opendir(".");

        dirfd(dir);

        return 0;
}