arrow

GH-41783: [C++] Make git-dependent definitions internal (#41781)

### Rationale for this change

Exposing the ARROW_GIT_ID and ARROW_GIT_DESCRIPTION preprocessor variables in our public headers tends to make incremental builds less efficient, since those values change very often during development.

Also, these values don't need to be preprocessor variables since they're just strings: you can't write useful `#if` directives with them. Instead, they can be inspected using `GetBuildInfo()`.

### Are these changes tested?

By existing builds and tests.

### Are there any user-facing changes?

Use cases depending on these preprocessor variables, which is unlikely, may break. They can be fixed by calling `arrow::GetBuildInfo()` instead.

* GitHub Issue: #41783

Authored-by: Antoine Pitrou <antoine@python.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>