I recently ported WebKit's libpas memory allocator[1] to Windows, which used pthreads on the Linux and Darwin ports. Depending on what pthreads features you're using it's not that much code to shim to Windows APIs. It's around ~200 LOC[2] for WebKit's usage, which a lot smaller than pthread-win32.

[1] https://github.com/WebKit/WebKit/pull/41945 [2] https://github.com/WebKit/WebKit/blob/main/Source/bmalloc/li...