missing makl_va_list_copy_by_value

[thomas-fossati]

It'd be nice to have a configure check to test whether va_list can be copied by value, e.g.:

#include <stdarg.h>
#include <stdlib.h>

void f(int i, ...)
{
  va_list args1, args2;
  va_start(args1, i);
  args2 = args1;
  if (va_arg(args2, int) != 42 || va_arg(args1, int) != 42)
    exit(1);
  va_end(args1);
  va_end(args2);
}

int main()
{
  f(0, 42);
  return 0;
}

This would be handy to conditionally disable stuff like https://github.com/koanlogic/klone/blob/master/src/libio/io.c#L637