CMake: add DART_USE_SYSTEM_GOOGLEBENCHMARK

[nim65s]

Hi,

This PR allow use of installed google benchmark instead of downloading it at configure time.

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Before creating a pull request

• Document new methods and classes
• Format new code files using ClangFormat by running make format
• Build with -DDART_TREAT_WARNINGS_AS_ERRORS=ON and resolve all the compile warnings

Before merging a pull request

• Set version target by selecting a milestone on the right side
• Summarize this change in CHANGELOG.md
• Add unit test(s) for this change
• Add Python bindings for new methods and classes

[jslee02]

I confirmed that this works with the option enabled and disabled. Thanks for implementing it!