Point CPM_SOURCE_CACHE at the vendored dependency cache

[avsej]

Motivation

The generated ext/cache/extconf_include.rb derives both
CPM_SOURCE_CACHE and
COUCHBASE_CXX_CLIENT_EMBED_MOZILLA_CA_BUNDLE_ROOT from __dir__, but
ext/extconf.rb evaluates that file through eval with no file name,
so __FILE__ is "(eval)" and __dir__ is nil. Both flags then
resolve against the working directory. Under gem install that
directory happens to be ext/, so the paths come out right by
accident; rake compile runs extconf.rb from Dir.tmpdir, where
they resolve to a cache under the temporary directory and the
dependencies and CA bundle vendored in ext/cache go unused.

Modifications

ext/extconf.rb passes the include file's path to eval, so __dir__
inside the snippet names the directory the snippet lives in, and the
generator emits that directory instead of expanding a relative cache
against it. The path is still computed when extconf runs rather than
when the gem is packaged, so a gem unpacked under any prefix finds its
own cache.

Results

Both flags name ext/cache whatever working directory the build is
driven from, so a source-tree build consumes the vendored dependencies
and the pinned CA bundle rather than fetching its own. A gem packaged
from a source tree and installed under a different prefix resolves both
flags to its own unpacked cache, so the vendored copy travels with the
gem.

[Copilot]

🟢 Approval recommended

The changes consistently resolve vendored paths independently of the build working directory.

Pull request overview

Fixes vendored dependency cache resolution when extconf.rb runs outside ext/.

Changes:

• Evaluates the generated include with its filename.
• Uses the include file’s directory for both cache paths.

File summaries

File	Description
Rakefile	Generates cache flags targeting ext/cache.
ext/extconf.rb	Supplies filename context to eval.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Balanced

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