Companion to https://github.com/Homebrew/brew/pull/7698.
Provide better, `odeprecated` messaging for
`depends_on :macos => :mavericks` and otherwise just fix up the code
that relied on `:mavericks`.
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.
