
This is a developer-only feature, so it's gated via `HOMEBREW_DEVELOPER`. This is intended to enable testing of macOS 10.15; users building software manually to test compatibility of early betas need to be able to build software from source instead of via pouring 10.14 bottles. This isn't intended to be a general-purpose `HOMEBREW_BUILD_FROM_SOURCE` replacement, and has no effect on released versions of macOS.