Make man page generation (pandoc) optional

Hello,

When I tried to compile generate the build file with cmake just to try the project I had to start by installing `pandoc`. On my system, at least, it's a bit annoying to do. It require to have a Haskell installation, which is not the smallest one and then `pandoc` takes a lot of time to compile (I decided to stop it in the middle because it was too long).

This pull request makes pandoc optional.

I understand the need of the man page if you want to install a "system wide" tool like this it's good to have documentation, and I will probably take the time to install pandoc someday. But for people who wants to give it a try I think it's a bit annoying, cmake generation + project compilation take less than 2 seconds to complete, installing pandoc require 'a lot' more.