test-strndup failure on AIX

[Lasse Collin]

test-strndup fails on AIX (cfarm119) with ibm-clang because
strndup(NULL, 0) == NULL isn't true. I had configured with CC=ibm-clang
CFLAGS=-O2. With -O0 it works.

The problem is that the strndup(NULL, 0) check in strndup.m4 is
optimized away, perhaps because the result isn't used. Testing on
Godbolt shows that x86-64 Clang >= 3.6 and GCC >= 15.1 do the same.
It's not due to the prototype in <string.h>, the NULL, or the 0,
because the following is optimized away too:

    #include <stddef.h>
    char *strndup(const char *s, size_t n);

    int main(void)
    {
      char buf[] = "abcd";
      int result = 0;

      if (strndup (NULL, 0) == NULL)
        result |= 2;

      if (strndup (buf, 0) == NULL)
        result |= 4;

      if (strndup (buf, 1) == NULL)
        result |= 8;

      if (strndup (buf, 3) == NULL)
        result |= 16;

      return result;
    }

This seems to work:

    char *volatile ptr;
    if ((ptr = strndup (NULL, 0)) == NULL)
      result |= 2;

-- 
Lasse Collin