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GH-40274: [C++] Add support for system glog 0.7 (#40275)

### Rationale for this change

glog uses "glog" not "GLOG" as CMake package name. So we should follow it.

### What changes are included in this PR?

Use "glogAlt" for our glog CMake module name to distinct upstream's CMake package name. 

### Are these changes tested?

No. We don't have CI with glog 0.7 yet.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #40274

Lead-authored-by: Sutou Kouhei <kou@clear-code.com>
Co-authored-by: Sutou Kouhei <kou@cozmixng.org>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>