use GCD semaphores on macOS

[zdAlexM]

On modern macOS, POSIX semaphores are unsupported. The API exists but always returns errno. Per a Stack Overflow solution, one can use Grand Central Dispatch as a drop-in replacement.

[eriklax]

After the latest change, this doesn't merge correctly with counter being moved up with error condtions.

[zdAlexM]

we moved the counter back down so it doesn't have to be duplicated in each ifdef, and also allows for distinguishing which part failed, the semaphor or the counter mmap.

[dfateyev]

Unlike POSIX semaphores with MAP_SHARED and pshared=1, GCD semaphores don't support fork() which is used there, so you would loose sync with children.
You may use a named semaphore and sem_open() which works with forking correctly and presents in macOS.