MacOS: fix usage of libdispatch and Availability macros in Thread

[barracuda156]

1. Fix usage of Availability macros: they should not be prefixed with underscores (those are used by internal headers, unprefixed work correctly always).
2. Do not assume libdispatch is always available, that is not the case. Use it where it actually exists.

[icraggs]

As far as I can see there is no alternative implementation if HAS_DISPATCH returns false. What happens then?

[barracuda156]

As far as I can see there is no alternative implementation if HAS_DISPATCH returns false. What happens then?

@icraggs If it is not defined, macro which checks for OSX and HAS_DISPATCH evaluates to false, and a generic posix fallback is used. Conditions under which it gets defined are specified in the header.
Do I miss something here?

I ran the build with and without enabling dispatch, both cases worked. (I have a custom libdispatch which is hidden from environment unless explicitly asked for.)

[icraggs]

The question is does the posix fallback work on MacOS? I remember now - MacOS doesn't support unnamed semaphores, so the fallback won't work. I think we'd need another section for MacOS without the dispatch calls.

[barracuda156]

@icraggs If you could help with switching that to Mach semaphores, that would be great.

I recall mentions about some issues with unnamed semaphores on macOS, though I probably did not encounter actual breakages due to them.