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GH-34670: [Packaging][C++] Add support for customizing GDB plugin install directory (#34672)

### Rationale for this change

It's for conda package. We can't determine the final GDB plugin path on building. We can adjust the path by replacing a placeholder on installing. 

### What changes are included in this PR?

Conda package can specify a placeholder for GDB plugin path.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* Closes: #34670

Authored-by: Sutou Kouhei <kou@clear-code.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>