how to reference variables in config [duplicate]

Question

Let's take undodir as an example; we might have something like this in our vimrc:

set undodir=$HOME/.config/nvim/undo/

In order to make config platform-agnostic one might want to use variables instead, eg

let conf_dir = stdpath('config')
set undodir=conf_dir.'/undo/'    " this is problematic

The above example doesn't work - conf_dir variable is not expanded as expected.