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

--- src/Makefile.in.orig	2008-03-09 16:51:35.000000000 -0400
+++ src/Makefile.in	2008-03-09 16:51:35.000000000 -0400
@@ -2,8 +2,7 @@
 #
 
 PROG	= tnftpd@EXEEXT@
-SRCS	= cmds.c conf.c ftpd.c ftpcmd.c popen.c @LSOBJS@
-# removed: logutmp.o logwtmp.o
+SRCS	= cmds.c conf.c ftpd.c ftpcmd.c popen.c logutmp.c logwtmp.c @LSOBJS@
 
 srcdir	= @srcdir@
 VPATH	= @srcdir@