[PATCH] [libc++] Allow libc++ to be built on systems without POSIX threads

[Dan Albert]

>From email thread:

>> Should we have a buildbot set up to build this configuration?
> I would appreciate having one, but I can't host it at the moment.
> At some point I would like to host a baremetal buildbot (targeting either QEMU, a raspberry pi, or some beagleboard), but I'm not quite ready for that... we need to work out the details of remote testing in lit before that is practical.

Keeping a buildbot going for this patch doesn't actually require baremetal (though that would be nice to have as well). @EricWF recently set up a buildbot. He might be willing to host it.

>> Would it be possible to define noop  implementations of all the things you've ifdef'd  out so that we don't have to worry about that? It's possible that would make things more or a mess, but I'm not sure.
> I think that nop-shims have an even higher potential for bit-rot than these #ifdef's.
> I originally attempted writing no-op shims for a few of them, but there were several where it didn't seem possible, and others where the shims were pretty hairy.

Okay, I thought that might be the case.

With those things accounted for, LGTM.

http://reviews.llvm.org/D3969