[CPP]. Optimizing Build Targets and Build Process

In the CPP project build, the following issues need to be addressed:

1. gtest should be introduced to the repository as a third-party compressed package, without the need for downloading during the build process.
2. Files that have already been downloaded should be checked to ensure they are not downloaded again unnecessarily.
3. CPP header files need to be placed under the include directory. Currently, header files are directly referenced from the source code directory, which is unreasonable.
4. Build scripts should include more interactive explanations.
5. Allow users to compile and produce artifacts with full static linking, dynamic linking, and compressed static linking.
6. Allow users to specify encoding compression dependencies during the build.

Start by inspecting the CPP project’s existing build scripts and source/header layout; the issue does not name specific files or tests. Map the dependency download behavior, header placement, build explanations, linking modes, and encoding-compression options, then verify that all six requested build improvements work without unnecessary downloads.

Written by the indexing model from the issue text.