After doing "rm -fr /usr/src" and then unpacking the tar_files
and fixing the unmatched "#endif" in ip_fil.h, I get:

cc   -o ipftest ipt.o ipft_sn.o ipft_ef.o ipft_td.o ipft_pc.o opt.o ipft_tx.o misc.o ip_frag.o ip_state.o ip_nat.o ipft_hx.o ip_fil.o fil.o parse.o ip_proxy.o 
ip_fil.o: Undefined symbol `_ipauth' referenced from text segment
ip_fil.o: Undefined symbol `_fr_auth_ioctl' referenced from text segment
ip_fil.o: Undefined symbol `_ipauth' referenced from text segment
ip_fil.o: Undefined symbol `_ipauth' referenced from text segment
ip_fil.o: Undefined symbol `_fr_auth_ioctl' referenced from text segment
fil.o: Undefined symbol `_fr_checkauth' referenced from text segment
fil.o: Undefined symbol `_fr_newauth' referenced from text segment
fil.o: Undefined symbol `_ipauth' referenced from text segment
fil.o: Undefined symbol `_fr_authstats' referenced from text segment
fil.o: Undefined symbol `_fr_authstats' referenced from text segment


What have I done wrong?

TIA,
Paul