Add checksum verification for libxml2, zlib, and zstd source archives via `cmake -E *sum` and `cmake -E compare_files` commands. This also adds the following minor changes: * Factor out libxml2 version into variable. * Check `tar` return code.
Add checksum verification for libxml2, zlib, and zstd source archives via `cmake -E *sum` and `cmake -E compare_files` commands. This also adds the following minor changes: * Factor out libxml2 version into variable. * Check `tar` return code.