Add allocator accepting constructor / fix pthreads on windows / add cmake support

[RedSkittleFox]

• CMake Support :
  • Updated readme
  • Multi-platform build action
  • max0x7ba::atomic_queue target
• Modified constructors of AtomicQueueB and AtomicQueueB2 to accept allocator
  • added get_allocator() methods
  • added unit tests
• Added checks for pthreads to fix build on windows

[max0x7ba]

Thank you for your massive contribution.

I'd rather you split your changes into 3 separate pull requests for ease of reviewing and merging:

1. CMake support. These can probably be merged with little changes. I am not a CMake user to be able to assess it by just eyeballing, will have to test drive your changes.
2. Windows compatibility changes. It should rather detect unavailability of pthread.h on Windows only, or just not include it at all when compiled on Windows. Platforms providing pthread.h shouldn't require any new preprocessor magic to maintain present behaviour.
3. Stateful allocators. That is something I'd need to review most thoughtfully.

Thank you.

[RedSkittleFox]

Split to 61 and 62 for now. Windows fix has to wait for 61 to be merged. https://github.com/max0x7ba/atomic_queue/pull/61 https://github.com/max0x7ba/atomic_queue/pull/62