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

  % gmake

Now you have spamc and spamd.