added a sys/queue.h implementation for Windows

[dplanitzer]

Added a copy of the Apple / Darwin sys/queue.h and hooked things up such that we use that one when building on Windows because the Windows SDK doesn't come with a sys/queue.h header.

[dplanitzer]

cc @MadCoder @compnerd

[MadCoder]

We can't do that this isn't the proper license.

@jrose-apple @jckarter @tkremenek what can we do here?

@dplanitzer is there a way to get it from somewhere else? e.g. on linux we use libbsd as a way to get this header from the "SDK".

[compnerd]

Why can't we use libbsd? I've been building against libbsd for Windows.

[adierking]

I've been working on adding Windows support to libbsd: https://gitlab.freedesktop.org/libbsd/libbsd/merge_requests/1

[tkremenek]

What precisely is the dependency here on libbsd? Is it just to get a header, or a link dependency? The license issue just needs to be reviewed (I can consult Apple's lawyers), but not necessarily a blocker.

[dplanitzer]

See my comment here. I can create an independent implementation of needed queue macros instead if that makes things simpler.

[tkremenek]

@dplanitzer I get a 404 with that link

[dplanitzer]

@tkremenek sorry about that. Fixed it. It's the last comment on the page.

[dplanitzer]

@MadCoder @tkremenek I've updated the PR with an independent implementation of the relevant TAILQ macros. Libdispatch only uses a handful of them anyway. I've tested the implementation locally and verified that it works beautifully :)

[MadCoder]

@swift-ci please test

[tkremenek]

@dplanitzer was the relevant implementation of TAILQ cribbed from the libbsd header? If so, technically that's copyrighted and can't just be copy-and-pasted into another place.

[dplanitzer]

@tkremenek I did a new implementation from scratch based on the documentation in the man 3 queue page. It's a simple standard doubly-linked list implementation while the sys/queue.h thing does something more fancy to avoid a few if-then checks on list manipulation.

[tkremenek]

@dplanitzer OK, perfect.

[tkremenek]

I think we can merge this.