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GH-33882: [C++] Don't find .pc files with ARROW_BUILD_STATIC=OFF (#34019)

### Rationale for this change

Because they are needless and `pkg-config grpc++` is slow.

### What changes are included in this PR?

Don't find .pc files with `ARROW_BUILD_STATIC=OFF`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

No.
* Closes: #33882

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