GH-47440: [C++] Accept gflags::gflags as system gflags CMake target (#47468)

### Rationale for this change

MSYS2's gflags package always uses `gflags::gflags` not `gflags::gflags_shared`.

### What changes are included in this PR?

Check `gflags::gflags`.

### Are these changes tested?

Yes.

### Are there any user-facing changes?

Yes.
* GitHub Issue: #47440

Authored-by: Sutou Kouhei <[email protected]>
Signed-off-by: Raúl Cumplido <[email protected]>

Author: kou

cpp/cmake_modules/ThirdpartyToolchain.cmake

@@ -1723,6 +1723,8 @@ if(ARROW_NEED_GFLAGS)
       set(GFLAGS_LIBRARIES gflags-shared)
     elseif(TARGET gflags_shared)
       set(GFLAGS_LIBRARIES gflags_shared)
+    elseif(TARGET gflags::gflags)
+      set(GFLAGS_LIBRARIES gflags::gflags)
     endif()
   endif()
 endif()