Fix:

error: implicit declaration of function 'yylex' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
error: implicit declaration of function 'yyerror' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
error: implicit declaration of function 'yyparse' is invalid in C99 [-Werror,-Wimplicit-function-declaration]
error: implicitly declaring library function 'exit' with type 'void (int) __attribute__((noreturn))' [-Werror,-Wimplicit-function-declaration]
error: implicitly declaring library function 'strdup' with type 'char *(const char *)' [-Werror,-Wimplicit-function-declaration]
error: type specifier missing, defaults to 'int'; ISO C99 and later do not support implicit int [-Wimplicit-int]

Includes changes from:

https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1066484
https://bugs.launchpad.net/bugs/2061589
--- configure.orig	2005-10-05 20:53:20.000000000 -0500
+++ configure	2024-08-02 21:48:39.000000000 -0500
@@ -3906,6 +3906,7 @@
 cat >>conftest.$ac_ext <<_ACEOF
 /* end confdefs.h.  */
 #include <ctype.h>
+#include <stdlib.h>
 #if ((' ' & 0x0FF) == 0x020)
 # define ISLOWER(c) ('a' <= (c) && (c) <= 'z')
 # define TOUPPER(c) (ISLOWER(c) ? 'A' + ((c) - 'a') : (c))
@@ -3919,7 +3920,7 @@
 
 #define XOR(e, f) (((e) && !(f)) || (!(e) && (f)))
 int
-main ()
+main (void)
 {
   int i;
   for (i = 0; i < 256; i++)
--- conf.h.orig
+++ conf.h
@@ -24,5 +24,8 @@
 #define CONF_H
 
 extern void config_type(char *, char *, char *, char *);
+int yyparse(void);
+int yyerror(char *s);
+int yylex(void);
 
 #endif /* CONF_H */
--- confl.l.orig
+++ confl.l
@@ -20,6 +20,7 @@
    Tcpxtract, a sniffer that extracts files based on headers
    by Nick Harbour
 */
+#include <stdlib.h>
 #include "confy.h"
 %}
 
--- confy.y.orig
+++ confy.y
@@ -23,6 +23,7 @@
 
 #include <stdlib.h>
 #include "conf.h"
+#include "confy.h"
 %}
 
 %union {
@@ -55,6 +56,7 @@
 
 %%
 #include <stdio.h>
+int
 yyerror(char *s)
 {
 	printf("%s\n", s);
--- tcpxtract.c.orig
+++ tcpxtract.c
@@ -44,6 +44,7 @@
 
 #include "sessionlist.h"
 #include "util.h"
+#include "conf.h"
 #include "confy.h"
 #include "search.h"