Add strndup for platforms that need it.

Some platforms don't have strndup, which includes Solaris 10, NetBSD 3
and FreeBSD 6.

configure.ac

@@ -1809,6 +1809,7 @@ AC_CHECK_FUNCS([ \
 	strlcat \
 	strlcpy \
 	strmode \
+	strndup \
 	strnlen \
 	strnvis \
 	strptime \

openbsd-compat/Makefile.in

@@ -51,6 +51,7 @@ OPENBSD=base64.o \
 	strlcat.o \
 	strlcpy.o \
 	strmode.o \
+	strndup.o \
 	strnlen.o \
 	strptime.o \
 	strsep.o \

openbsd-compat/strndup.c

@@ -0,0 +1,43 @@
+/*	$OpenBSD: strndup.c,v 1.2 2015/08/31 02:53:57 guenther Exp $	*/
+
+/*
+ * Copyright (c) 2010 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 "config.h"
+#ifndef HAVE_STRNDUP
+#include <sys/types.h>
+
+#include <stddef.h>
+#include <stdlib.h>
+#include <string.h>
+
+char *
+strndup(const char *str, size_t maxlen)
+{
+	char *copy;
+	size_t len;
+
+	len = strnlen(str, maxlen);
+	copy = malloc(len + 1);
+	if (copy != NULL) {
+		(void)memcpy(copy, str, len);
+		copy[len] = '\0';
+	}
+
+	return copy;
+}
+DEF_WEAK(strndup);
+#endif  /* HAVE_STRNDUP */