FreeBSD does have support for utmp/wtmp, so include relevant files into
a build.

--- src/Makefile.in.orig	Wed Dec 13 13:03:12 2006
+++ src/Makefile.in	Wed Dec 13 13:03:47 2006
@@ -23,8 +23,7 @@
 INSTALL	= @INSTALL@
 
 PROG	= tnftpd
-OBJS	= cmds.o conf.o ftpd.o ftpcmd.o popen.o @LSOBJS@
-# removed: logutmp.o logwtmp.o
+OBJS	= cmds.o conf.o ftpd.o ftpcmd.o popen.o logutmp.o logwtmp.o @LSOBJS@
 
 all:	${PROG}