glibc (2.38)

(root)/
include/
bits/
a.out.h
       1  #ifndef __A_OUT_GNU_H__
       2  # error "Never use <bits/a.out.h> directly; include <a.out.h> instead."
       3  #endif
       4  
       5  #ifdef __x86_64__
       6  
       7  /* Signal to users of this header that this architecture really doesn't
       8     support a.out binary format.  */
       9  #define __NO_A_OUT_SUPPORT 1
      10  
      11  #endif