glibc has two strerror_r functions, declared in <string.h> depending on
feature macros. But none of them is POSIX compliant.

The one which is in effect when _GNU_SOURCE=1 has the declaration
char *strerror_r (int, char *, size_t). It has a different return type
and therefore a different calling convention than the POSIX function.
Test case:
=================================================
#define _GNU_SOURCE 1
#include <string.h>
#include <stdio.h>

int main()
{
  char buf[100];
  char *s = strerror_r (-2, buf, sizeof (buf));
  printf ("result: %s\n", s);
  return 0;
}
=================================================

The one which is in effect when _GNU_SOURCE is undefined and
_POSIX_C_SOURCE=200112L has the declaration
int strerror_r (int, char *, size_t), which is POSIX compatible,
but it has a different return value convention in case of failure.
POSIX:2001 and POSIX:2008 say that in case of failure the strerror_r
function should return "an error number"; this is the same wording
as for pthread_create, pthread_mutex_lock, etc. However, the glibc
function returns -1 and sets errno to the error number instead.
Test case:
========================================================================
#define _POSIX_C_SOURCE 200112L
#include <string.h>
#include <errno.h>
#include <stdio.h>

int main()
{
  char buf[100] = "______";
  int r;

  r = strerror_r (-2, buf, sizeof (buf));
  if (r == 0)
    printf ("result: success %s\n", buf);
  else if (r > 0)
    printf ("result: failure %d %s\n", r, buf);
  else
    printf ("result: non-POSIX failure %d %d %s\n", r, errno, buf);

  r = strerror_r (EACCES, buf, 3);
  if (r == 0)
    printf ("result: success %s\n", buf);
  else if (r > 0)
    printf ("result: failure %d %s\n", r, buf);
  else
    printf ("result: non-POSIX failure %d %d %s\n", r, errno, buf);

  return 0;
}
==========================================================================
Expected result:
result: failure 22 ______
result: failure 34 ______

Actual result:
result: non-POSIX failure -1 22 ______
result: non-POSIX failure -1 34 ______