Mike Nolan <nolan AT naic dot edu> has built this, and here's his directions:
Get the library containing a SunOS version of snprintf, available at http://SpamAssassin.org/released/contrib/ .
I built SpamAssassin under SunOS 4.1.3.
% gtar xzf snp.tar.gz
% cd snp
% gmake libsnp.a
OK, now go to the spamassassin directory, and compile:
% cd ..
% gcc -g -O2 spamd/spamc.c spamd/libspamc.c spamd/utils.c -o spamd/spamc -ldl -lnsl snp/libsnp.a -lm
Now you have spamc and spamd.