Support building with MSVC

[DomClark]

See also LMMS/lmms#6561.

This pull request mostly consists of routine transformations from POSIX APIs to the C++ standard library, along with _alloca alternatives for variable-length stack arrays. In order to keep things easy to review, and maybe pull in other upstream changes, I haven't tried to improve the code by using things such as RAII lock guards (except where required, i.e. in conjunction with a condition variable). As an exception, I took the opportunity to support Unicode paths in a few places by using wide character functions on Windows.

[JohannesLorenz]

👍

Many parts of this PR might not be applied to current zyn's master (e.g. some variables do not occur there, and we meanwhile also have the STACKALLOC macro for stack allocation), but a few could (e.g. the mkdir changes). Are you going to push those parts that can be applied to zyn's master, too?

[DomClark]

Are you going to push those parts that can be applied to zyn's master, too?

This is something I definitely plan to do eventually, especially since we plan on updating our Zyn version and presumably still want to be able to build everything with MSVC. Right now, this is mostly for personal convenience as I'm working on our remote plugin IPC interface and want to be able to test my changes with Zyn more easily.

[PhysSong]

In a second look, I noticed that ThreadShims.h is outside this repository. Is it safe to do such a thing?

[PhysSong]

Oh, We already do that for IoHelper.h.

[DomClark]

Yeah, it's not ideal, but I needed it in the LMMS repository too, and didn't want to duplicate it. As you say, we already do the same for IoHelper.h, and hopefully we can get rid of it soon once we can use a MinGW version with built-in support.