This is the strlcat function from openbsd:
http://www.openbsd.org/cgi-bin/cvsweb/~checkout~/src/lib/libc/string/strlcat.c

It is of course BSD-licensed, just like milter-regex itself.

--- /dev/null	2010-08-02 01:13:15.113045120 +0100
+++ strlcat.c	2005-08-08 09:05:37.000000000 +0100
@@ -0,0 +1,55 @@
+/*	$OpenBSD: strlcat.c,v 1.13 2005/08/08 08:05:37 espie Exp $	*/
+
+/*
+ * Copyright (c) 1998 Todd C. Miller <Todd.Miller@courtesan.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+
+#include <sys/types.h>
+#include <string.h>
+
+/*
+ * Appends src to string dst of size siz (unlike strncat, siz is the
+ * full size of dst, not space left).  At most siz-1 characters
+ * will be copied.  Always NUL terminates (unless siz <= strlen(dst)).
+ * Returns strlen(src) + MIN(siz, strlen(initial dst)).
+ * If retval >= siz, truncation occurred.
+ */
+size_t
+strlcat(char *dst, const char *src, size_t siz)
+{
+	char *d = dst;
+	const char *s = src;
+	size_t n = siz;
+	size_t dlen;
+
+	/* Find the end of dst and adjust bytes left but don't go past end */
+	while (n-- != 0 && *d != '\0')
+		d++;
+	dlen = d - dst;
+	n = siz - dlen;
+
+	if (n == 0)
+		return(dlen + strlen(s));
+	while (*s != '\0') {
+		if (n != 1) {
+			*d++ = *s;
+			n--;
+		}
+		s++;
+	}
+	*d = '\0';
+
+	return(dlen + (s - src));	/* count does not include NUL */
+}
--- Makefile.linux.orig	2007-01-11 15:49:52.000000000 +0000
+++ Makefile.linux	2010-08-13 09:55:38.047330128 +0100
@@ -11,8 +11,8 @@
 
 all: milter-regex milter-regex.cat8
 
-milter-regex: milter-regex.o eval.o strlcpy.o y.tab.o
-	gcc -o milter-regex milter-regex.o eval.o strlcpy.o y.tab.o $(LDFLAGS)
+milter-regex: milter-regex.o eval.o strlcat.o strlcpy.o y.tab.o
+	gcc -o milter-regex milter-regex.o eval.o strlcat.o strlcpy.o y.tab.o $(LDFLAGS)
 
 milter-regex.o: milter-regex.c eval.h
 	gcc $(CFLAGS) -c milter-regex.c
@@ -20,6 +20,9 @@
 eval.o: eval.c eval.h
 	gcc $(CFLAGS) -c eval.c
 
+strlcat.o: strlcat.c
+	gcc $(CFLAGS) -c strlcat.c
+	
 strlcpy.o: strlcpy.c
 	gcc $(CFLAGS) -c strlcpy.c
 	
--- milter-regex.c.orig	2010-08-13 09:18:07.433332449 +0100
+++ milter-regex.c	2010-08-13 09:58:12.098393196 +0100
@@ -135,6 +135,7 @@
 
 #if __linux__ || __sun__
 #define	ST_MTIME st_mtime
+extern size_t	 strlcat(char *, const char *, size_t);
 extern size_t	 strlcpy(char *, const char *, size_t);
 #else
 #define	ST_MTIME st_mtimespec