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This patch adds rudimentary windows build support. A complete platform implementation is not included, so not all targets are added to `gn_all` yet. Bug: 236163082 Change-Id: I4f6d81bb0babb56a94e7c24777e4c572714d1ade Reviewed-on: https://chromium-review.googlesource.com/c/openscreen/+/5490382 Commit-Queue: Jordan Bayles <jophba@chromium.org> Reviewed-by: Mark Foltz <mfoltz@chromium.org>