Remove build warnings

[YanNoun]

This PR removes all the annoying build warnings that we had by :

• Setting CMake policy
• Add C++ warning ignores in third-party libraries only, since we're very unlikely to touch this code ever again
• Fix the actual warnings in the rest of OpenSfM code

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@fabianschenk has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

[facebook-github-bot]

@fabianschenk merged this pull request in 752e008.