Prefer find_package(GTest) over FetchContent

[Copilot]

GTest was always downloaded via FetchContent, even when system-installed. This change prefers find_package(GTest) and falls back to FetchContent only when unavailable.

Changes

• Add find_package(GTest QUIET) before FetchContent fallback
• Add CSR4MPI_FORCE_FETCH_GTEST option to bypass find_package when needed
• Check both GTest_FOUND and GTEST_FOUND for compatibility with different CMake configs
• Add DOWNLOAD_EXTRACT_TIMESTAMP TRUE to silence CMP0135 warning

Usage

# Use system GTest (default)
cmake -G Ninja ..

# Force FetchContent
cmake -G Ninja -DCSR4MPI_FORCE_FETCH_GTEST=ON ..

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

关闭对 gtest 的强行集成，优先使用 cmake 的 find_package 查找

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