extconf: respect AR and RANLIB in recipes on darwin

[joshheinrichs-shopify]

What problem is this PR intended to solve?

When using Nix on Darwin, we ideally should be latching onto the Nix-provided AR and RANLIB. Without this, when blocking Xcode via sandbox-exec, I see errors like this while attempting to install the gem, since we end up using the default ar and runlib

xcode-select: note: No developer tools were found, requesting install.
If developer tools are located at a non-default location on disk, use `sudo xcode-select --switch path/to/Xcode.app` to specify the Xcode that you wish to use for command line developer tools, and cancel the installation dialog.
See `man xcode-select` for more details.

Have you included adequate test coverage?

No but this seems relatively harmless?

Does this change affect the behavior of either the C or the Java implementations?

No