makefile: prevent hypothetical double expansion

Commit message:

> Given the following makefile:
>
> ```make
> X = $(shell echo '$$HOME')
>
> example:
>     @echo 'single-quoted: $(X)'
>     @echo "double-quoted: $(X)"
> ```
>
> Running `make` will output something similar to the following:
>
> ```
> single-quoted: $HOME
> double-quoted: /Users/dc
> ```
>
> Though double expansion will almost certainly never occur in practice, there's no downside to correctness in this case.

This project provides an excellent example of a makefile for a Node package. This is the reason I consider such trivial improvements worthwhile. :)