Make server command issue threadpoool/threadpool.h

I am having this issue when I write "make server" command.

Yes, with git clone ... the folder is empty. I download it with by:

cd webserver
git clone https://github.com/pminkov/threadpool

The prints in cgi-bin also have to be updated (assuming we're using python3 now). I just made pull requests with these changes and updated them in my branch. :)