Well, I spent 5 minutes and I got 1 line further..
firewalk check if pcap_open_live function exists in the lib pcap.
in config.log
gcc -o conftest -g -O2 -Wall conftest.c -lpcap -lnet
You should look at the message after that.
the line where it doesn't work is that is looks for libdnet which is.. nowadays decnet and not libnet. And anyway the current libnet doesn't have the function its looking for.
I don't understand why you are stuck on the line before though. And don't move files, you're just creating more problems. This one should work.
Either you take libdnet from the time when firewalk was released (2002..) or you take another tool like hping3, apt-get install hping3 and have fun reading the manual
Sorry I have no more time.