Originally Homebrew was a build-from-source package manager and all user environment variables and non-Homebrew-installed software were available to builds. Since then Homebrew added Requirements to specify dependencies on non-Homebrew software (such as those provided by brew cask like X11/XQuartz), the superenv build system to strip out unspecified dependencies, environment filtering to stop the user environment leaking into Homebrew builds and default_formula to specify that a Requirement can be satisfied by a particular formula.
As Homebrew became primarily a binary package manager, most users were fulfilling Requirements with the default_formula, not with arbitrary alternatives. To improve quality and reduce variation, Homebrew now exclusively supports using the default formula, as an ordinary dependency, and no longer supports using arbitrary alternatives.
If you wish to build against custom non-Homebrew dependencies that are provided by Homebrew (e.g. a non-Homebrew, non-macOS ruby) then you must create and maintain your own tap as these formulae will not be accepted in Homebrew/homebrew-core. Once you have done that you can specify env :std in the formula which will allow e.g. which ruby to access your existing PATH variable and allow compilation to link against this Ruby. You can also include a custom Requirement in your formula that more accurately describes the non-Homebrew software you build against.
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https://docs.brew.sh/Building-Against-Non-Homebrew-Dependencies