This is the same problem on IRIX. You have to use the flex library like
this:
gcc ... -L(flex directory path) -lfl

Gabor

sorry to say but Gabor's advice didn't bring me any further. What I chose then was to set

export LEXLIB="/opt/sfw/libfl.a"

since this file holds yywrap. I made a

make disclean

./configure

make

and everything went fine (at least there were no errors, warnings, notices or other complaints reported) and binaries were ceated.

fyi we use:

gcc 3.3

flex 2.5.4

libol 0.3.11

syslog-ng 1.6.0rc4

now the config will begin.