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GH-34861: [C++] Move GTest std version check into SYSTEM if branch (#34874)

### Rationale for this change
Developers have reported that the current GTest std version check will break cpp microbenchmarks in BUNDLE mode.

### What changes are included in this PR?
Move the check into SYSTEM if branch so that it gets triggered in a narrowed scope.

### Are these changes tested?
Tested locally and will be covered by CI.

### Are there any user-facing changes?
No

* Closes: #34861

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