xmalloc calling undeclared xalloc_die function

[Florian Weimer]

lib/xmalloc.c contains this function definition, unconditionally:

  static void * _GL_ATTRIBUTE_PURE
  nonnull (void *p)
  {
    if (!p)
      xalloc_die ();
    return p;
  }

But the declaration of xalloc_die in lib/xalloc.h is conditional:

  #if GNULIB_XALLOC_DIE
  
  /* This function is always triggered when memory is exhausted.
     It must be defined by the application, either explicitly
     or by using gnulib's xalloc-die module.  This is the
     function to call when one wants the program to die because of a
     memory allocation failure.  */
  /*extern*/ _Noreturn void xalloc_die (void);
  
  #endif /* GNULIB_XALLOC_DIE */

I have a package (lbzip2 <https://github.com/kjn/lbzip2/>) which
supplies its own definition of xalloc_die, and fails to build due to an
undeclared function.  So I'm wondering how this is supposed to work.

Thanks,
Florian