Add CPack support

[zjyhjqs]

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1. Add PROJECT_IS_TOP_LEVEL variable, to decide whether need to install/package
2. Add the main information of project() that CPack uses
3. Set the package name (GTest) as same as the name in find_package
4. Enable the CPack functionality

The package will be: i.e. GTest-1.15.2-win64.tar.gz

Ref: #2530 (I'm not familiar with RPM/DEB, so I just enable the basic CPack functionalities.)

[zjyhjqs]

Please review the PR and merge it if possible.