Windows support (WiP)

[SamuelMarks]

Here's my WiP - https://github.com/SamuelMarks/s2n-tls/tree/windows

Problem:

You don't support Windows

Solution:

Add support for Windows.

• Does this change what S2N sends over the wire? Nah
• Does this change any public APIs? Maybe a little for standard UNIX/Linux headers now exposed functions on Windows
• Which versions of TLS will this impact? N/A

Requirements / Acceptance Criteria:

What must a solution address in order to solve the problem? How do we know the solution is complete? - When it builds for Windows (and the test pass I guess!)

• Related Issues: Add support for a Windows Build #497

Out of scope:

Is there anything the solution will intentionally NOT address?

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So yeah anyway if you want me to keep going just say the word and I'll see if I can't get it as far as successful build. Might even try for all of MSVC 2022, Cygwin, and MinGW.

[camshaft]

If you can get it building, we'd be more than happy to accept the PR! One thing you'll probably run into, though, is we use cleanup attributes quite extensively, which last I checked were not supported by MSVC.

[SamuelMarks]

Ah yes, this thing:

#define DEFER_CLEANUP(_thealloc, _thecleanup) \
    __attribute__((cleanup(_thecleanup))) _thealloc

It's a bit hacky but I'm confident macros could solve the issue. Also there are only 1381 odd usages, so shouldn't be more than an afternoon to resolve issues.

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Cool now that I know you're interested I'll put some time to this PR and have it for you ASAP.