libio: Assume _LIBC, weak_alias, errno, (__set_)errno &c are defined

Do not define _POSIX_SOURCE.

libio/iofgetpos64.c

@@ -47,10 +47,8 @@ _IO_new_fgetpos64 (_IO_FILE *fp, _IO_fpos64_t *posp)
     {
       /* ANSI explicitly requires setting errno to a positive value on
 	 failure.  */
-# ifdef EIO
       if (errno == 0)
 	__set_errno (EIO);
-# endif
       result = EOF;
     }
   else