include/nuttx/compiler.h, include/poll.h: Add logic to determine if the compiler supports anonymous structures and unions.

include/poll.h

@@ -116,11 +116,16 @@ struct pollfd
    * they are not available in C89.
    */
 
+#ifdef CONFIG_HAVE_ANONYMOUS_UNION
   union
   {
     int        fd;      /* The descriptor being polled */
     FAR void  *ptr;     /* The psock or file being polled */
   };
+#else
+  int          fd;      /* The descriptor being polled */
+  FAR void    *ptr;     /* The psock or file being polled */
+#endif
 
   FAR sem_t   *sem;     /* Pointer to semaphore used to post output event */
   pollevent_t  events;  /* The input event flags */