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

--- src/Makefile.am.orig	2010-01-04 06:46:29.000000000 +0100
+++ src/Makefile.am	2010-01-04 06:46:29.000000000 +0100
@@ -7,7 +7,9 @@
 	conf.c \
 	ftpd.c \
 	ftpcmd.y \
-	popen.c
+	popen.c \
+	logutmp.c \
+	logwtmp.c
 
 AM_CPPFLAGS = \
 	-DHAVE_TNFTPD_H=1 \
@@ -45,8 +47,6 @@
 	extern.h \
 	ftpd.conf.manin \
 	ftpusers.manin \
-	logutmp.c \
-	logwtmp.c \
 	pathnames.h \
 	tnftpd.manin \
 	version.h