gtest: add cmake support

[star-hengxing]

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[gemini-code-assist]

Summary of Changes

Hello @star-hengxing, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly improves the flexibility of the gtest package by integrating CMake as an alternative build system. This change allows users to leverage CMake for building gtest, providing more options for dependency management and build customization, while maintaining compatibility with the existing xmake-driven build process. The primary impact is offering a choice in how gtest is compiled and installed within the xmake ecosystem.

Highlights

• New Build System Option: Introduced a new configuration option, cmake, for the gtest package, allowing users to explicitly choose between xmake's native build system and CMake for building gtest.
• Conditional CMake Dependency: Added a conditional dependency on the cmake package, which is automatically included if the cmake build system option is enabled for gtest.
• Flexible Installation Logic: Modified the on_install function to dynamically switch between CMake-based installation and the existing xmake-based build process, depending on the cmake configuration. This includes passing relevant build parameters like build type, shared library status, and gmock support to CMake.

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[gemini-code-assist]

Code Review

This pull request introduces CMake support for the gtest package, making it the default build method. This is a positive change that aligns the package with gtest's official build system. The implementation is largely correct, adding the necessary CMake dependency and configuring the build appropriately. However, there is a high-severity issue where the existing main configuration option is ignored in the new CMake build path, leading to inconsistent behavior. I've also included a medium-severity suggestion to improve code conciseness.

[gemini-code-assist]

The new CMake build path does not respect the main configuration option. In the xmake build path, package:config("main") controls whether the gtest_main target is a default part of the package. In this new CMake path, the main config is ignored, and gtest_main is always built but never becomes a default target. This is a behavioral regression that breaks configuration consistency for the package. Please ensure the main config is also handled in the CMake build path to maintain consistent behavior between the two build options.