Fix build on FreeBSD (and unix)

[lbartoletti]

Hi,

Thanks for this project!

I propose a PR to build jupyternim on FreeBSD (and other unix like), since the traditional path is the same on linux and other unix.

[stisa]

Hi! Thanks for the PR. Looks good to me.
Do you know where I could find a list of defines? I found https://github.com/nim-lang/Nim/wiki/Consts-defined-by-the-compiler but unix isn't on it. I know it works and I've seen it in the stdlib code, but I can't find where it gets set, it's been a while since I read the compiler code

[lbartoletti]

I guess here