For me AC_CHECK_FUNC expands to

char closesocket ();
int main()
{
  return closesocket ();
}

which should fail to link if closesocket is not available.  Maybe your
autoconf is too old, not doing a function call but doing sth else?

Indeed:

/* System header to define __stub macros and hopefully few prototypes,
    which can conflict with char $ac_func (); below.  */
#include <assert.h>
/* Override any gcc2 internal prototype to avoid an error.  */
#ifdef __cplusplus
extern "C"
#endif
/* We use char because int might match the return type of a gcc2
   builtin and then its argument prototype would still apply.  */
char $ac_func ();
char (*f) ();

#ifdef F77_DUMMY_MAIN
#  ifdef __cplusplus
     extern "C"
#  endif
   int F77_DUMMY_MAIN() { return 1; }
#endif
int
main ()
{
/* The GNU C library defines this for functions which it implements
    to always fail with ENOSYS.  Some functions are actually named
    something starting with __ and the normal name is an alias.  */
#if defined (__stub_$ac_func) || defined (__stub___$ac_func)
choke me
#else
f = $ac_func;
#endif

  ;
  return 0;
}


the global variable 'f' is unused and thus elided by -flto, removing the
reference to the function we want to check for.  Making 'f' volatile
would also help.

char (* volatile f) ();

in particular.

Any progress on that one ?

I found that the current autoconf can handle this. However, fwnn delivers an old configure script that is just too old.

Created sr #725078 to update the fwnn package to use autoreconf -fi which lets the package build again.

This bug is resolved imho.