Can not compile net/tnftpd

[Martin Husemann <martin%duskware.de@localhost>]

On a "slightly" older SuSE Linux I am trying to install net/tnfptd, but
compilation fails due to yacc producing bogus code:

yacc  ftpcmd.y
gcc -I. -I./.. -I. -I..  -I./../libnetbsd -DHAVE_TNFTPD_H=1 -O2 
-D_DEFAULT_CONFDIR=\"/usr/pkg/etc\"  -c -o ftpcmd.o y.tab.c
ftpcmd.y: In function `yyparse':
ftpcmd.y:300: error: `epsvall' undeclared (first use in this function)
ftpcmd.y:300: error: (Each undeclared identifier is reported only once
ftpcmd.y:300: error: for each function it appears in.)
ftpcmd.y:544: error: `sitetab' undeclared (first use in this function)
ftpcmd.y: At top level:
ftpcmd.y:1293: error: `sitetab' used prior to declaration
ftpcmd.y:1303: warning: `check_write' was declared implicitly `extern' and 
later `static'
ftpcmd.y:896: warning: previous declaration of `check_write'
ftpcmd.y:1304: warning: `help' was declared implicitly `extern' and later 
`static'
ftpcmd.y:732: warning: previous declaration of `help'
ftpcmd.y:1304: warning: type mismatch with previous implicit declaration
ftpcmd.y:732: warning: previous implicit declaration of `help'
ftpcmd.y:1304: warning: `help' was previously implicitly declared to return 
`int'

(and some more).

Yacc is invoking /usr/pkg/bin/bison -y "$@", and that bison is

bison-2.3           GNU yacc(1) replacement

What am I doing wrong?

Martin