Fix ruby/io-console#105.

Reads better to me:

> checking prerequisites for ZJIT... yes

There are many indentation styles for AS_CASE in this file but no one
uses `[*]` for the default case.

So we don't expose builders to network flakiness which cannot be worked
around using cargo's --offline flag.

"Default" means when `--enable-zjit` is absent from `./configure`
arguments.

Building ZJIT requires GNU make at the moment.
To get access to `$gnumake`, lift the `make` flavour detection up to the
environment section, before the JIT section runs.

This runs the detection, but does nothing with the result.

 * Fixed version requirement in messages -- ZJIT requires >= 1.85 unlike
  YJIT.
 * New: Detect when rust 1.85 is available, and neither --enable-yjit nor
  --enable-zjit is passed to ./configure, include both YJIT and ZJIT
  in the build

When relative loading, `prefix` makes no sense actually.  Use the
given (or default) path as `DESTDIR` instead.

This change affects only when the relative loading is enabled and the
destdir is not given, and does not change the final installation path,
but makes the configuration options simpler a little.