#define FTP_NAMES
#include 
の下に１行を追加します
#include "../support/ftp.h" 

これを追加しないでbuild s41を実行すると以下のエラーがです。

Making ftpd.
gcc -g -DDEBUG -I.. -I../support -L../support   -c ftpd.c -o
ftpd.o
ftpd.c: In function `statcmd':
ftpd.c:1973: `typenames' undeclared (first use this function)
ftpd.c:1973: (Each undeclared identifier is reported only once
ftpd.c:1973: for each function it appears in.)
ftpd.c:1975: `formnames' undeclared (first use this function)
ftpd.c:1983: `strunames' undeclared (first use this function)
ftpd.c:1983: `modenames' undeclared (first use this function)
make: *** [ftpd.o] Error 1