Make pango and fontconfig optional on *nix.

I'm mostly opening this pull request as a way to start discussion.

I don't think this is necessarily the best way to do this, because it makes the library inconsistent. For example, if someone distributes a script that depends on these libraries to be installed, their users will probably see very cryptic error messages.

A better way may be, instead of replacing `create_font` and `list_fonts` with `nil`, replace them with something like:
```lua
function()
  error('You are missing the library required to use this function');
end
```

Other ideas welcome.