[Exim] exim whoson compile error

I've been trying to compile exim with LOOKUP_WHOSON but I invariably get a
compile error:

gcc -o exim  acl.o child.o crypt16.o daemon.o dbfn.o debug.o deliver.o
directory.o dns.o drtables.o enq.o exim.o expand.o filter.o filtertest.o
globals.o header.o host.o ip.o log.o lss.o match.o moan.o os.o parse.o
queue.o rda.o readconf.o receive.o retry.o rewrite.o rfc2047.o route.o
search.o sieve.o smtp_in.o smtp_out.o spool_in.o spool_out.o store.o
string.o tls.o tod.o transport.o tree.o verify.o local_scan.o  version.o \
  pcre/libpcre.a \
  routers/routers.a transports/transports.a lookups/lookups.a \
  auths/auths.a \
  -lresolv -lnsl -lcrypt    \
   -ldb
lookups/lookups.a(whoson.o)(.text+0x1a): In function `whoson_find':
: undefined reference to `wso_query'
collect2: ld returned 1 exit status
make[1]: *** [exim] Error 1
make[1]: Leaving directory `/usr/local/src/exim-4.31/build-Linux-i386'
make: *** [go] Error 2

exim-4.31
whoson-2.03
gcc version 3.2.3

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