Make a link-time warning for the use of gets().

IMHO, the run-time warning should come out, but I'm not game to start that
fight yet...  This uses a feature of the gnu linker.

Inspired by:  NetBSD

lib/libc/stdio/gets.c

@@ -40,6 +40,9 @@ static char sccsid[] = "@(#)gets.c	8.1 (Berkeley) 6/4/93";
 
 #include <unistd.h>
 #include <stdio.h>
+#include <sys/cdefs.h>
+
+__warn_references(gets, "warning: this program uses gets(), which is unsafe.");
 
 char *
 gets(buf)