Compensate for platforms that don't have strndup, like Solaris

--- configure.in
+++ configure.in
@@ -30,7 +30,7 @@
 dnl Checks for library functions.
 AC_TYPE_SIGNAL
 AC_FUNC_VPRINTF
-AC_CHECK_FUNCS(gethostname socket strdup strstr)
+AC_CHECK_FUNCS(gethostname socket strdup strndup strstr)
 
 dnl Check for optional features
 AC_ARG_ENABLE(logfile, 
--- ssmtp.c
+++ ssmtp.c
@@ -842,7 +842,16 @@
 	if (!rest) {
 		return NULL;
 	}
+#ifdef HAVE_STRNDUP
 	tok=strndup(*s,rest-(*s));
+#else
+	{
+		size_t len = rest - (*s);
+		tok = malloc(sizeof(char) * (len + 1));
+		memcpy(tok, *s, len);
+		tok[len] = '\0';
+	}
+#endif
 	if (!tok) {
 		die("firsttok() -- strndup() failed");
 	}