Add platform guards for Windows APIs.

[bitbound]

The Windows project no longer explicitly targets Windows, so platform guards are needed to prevent warnings. I also set this warning to be treated as an error.

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Thank you for your contribution to the Remotely project. It is required that contributors assign copyright to Immense Networks so we retain full ownership of the project.

This makes it easier for other entities to use the software because they only have to deal with one copyright holder. It also gives me assurance that we'll be able to make decisions in the future without gathering and consulting all contributors.

While this may seem odd, many open source maintainers practice this. Here are a couple well-known examples:

• Free Software Foundation: http://www.gnu.org/licenses/why-assign.html
• Microsoft: https://cla.opensource.microsoft.com/

A nice article on the topic can be found here: https://haacked.com/archive/2006/01/26/WhoOwnstheCopyrightforAnOpenSourceProject.aspx/

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