How can I prevent Homebrew from installing a duplicate of software I installed through other means?

Question

I like using Homebrew for installing Unix software on my Mac, and appreciate that it doesn't try to install multiple copies of things like Python. There are some packages I've already installed manually, though, and I don't want Homebrew to install its own copies of them.

In my current situation, I installed OCaml using the standard installer. Now I want to install Coq via Homebrew. I see via brew deps coq that it depends on OCaml. I know I can tell it to skip installing OCaml this time via brew install --ignore-dependencies coq. However, I'd rather not have to do that every time I install a Homebrew package that requires OCaml.

How can I make Homebrew pretend that it installed software that I installed through other means, so that it doesn't try to install a duplicate copy?