As pointed out by Michiel Thuys <michiel.thuys@intersil.com>, this was

checking sizeof which was pure bloat and in many cases wrong.
 -Erik

libc/sysdeps/linux/common/seteuid.c

@@ -4,19 +4,5 @@
 
 int seteuid(uid_t uid)
 {
-  switch (sizeof (uid_t))
-  {
-  case 2:
-    if (uid == 65535)
-    {
-      __set_errno(EINVAL);
-      return -1;
-    }
-    break;
-
-  default:
-    fprintf (stderr, "Uknown uid_t size and sign\n");
-  }
-
-  return setreuid(-1, uid);
+    return setreuid(-1, uid);
 }