Please support Mingw-w64 on Windows

[rock59]

It would be nice to add MinGW support in cmakelists and provide MSYS2 package for users who need PicoTLS in Windows

[huitema]

You can get Picotls on Windows with the Visual Studio project "picotlsvs.sln". I use it regularly.

[rock59]

Building PicoTLS with VS and using the generated DLL in MinGW is easy but it's subject to VS licensing limits
I am porting H2O to windows uisng MinGW and wepoll (https://github.com/piscisaureus/wepoll) and I need PicoTLS to support HTTP/3

[syoyo]

@rock59 I made experimental MinGW build of picotls(minicrypto backend)

https://github.com/syoyo/picotls/tree/mingw-build

https://github.com/syoyo/picotls/blob/mingw-build/MingwPort.md

After getting success to compile picotls using MinGW with OpenSSL, I am planning to send a PR.

FYI, if you just need to compile picotls using MSVC, you can use C++ Build Tools(Build Tools for Visual Studio 2019, which is free to use) + cmake + ninja

https://visualstudio.microsoft.com/downloads/

(Some cmake modification to picotls will be required though)

[rock59]

@syoyo Great job. Thanks