Fix build for clang

[Patater]

Remove declaring _POSIX_SOURCE manually. Let the compiler take care of this on its own where it can. Additionally, include a missing header file for the definition of struct sockaddr_in, which we need before we can allocate the struct. With these fixes, we can build with Clang 18.1.5, although there are many warnings.

[Patater]

Build failed on Windows. I've updated the commit to restore the _POSIX_SOURCE defines when building for Windows.

[Patater]

Rebased on latest master