C++11 std mutex and condition variable

[Aidan63]

Replaces the old implementation which had a bunch of ifdefs depending on the platform. Now we have a single implementation using the C++ std types. I would have done the same for Lock but for some reason it took until C++20 to get a semaphore type...

I have broken the definitions out into their own headers and exposed the classes so on the haxe side we can remove the untyped code.

[tobil4sk]

Could we omit these for hxcpp_api_level >= 500? It would be good to deprecate them anyway since identifiers containing __ are reserved by c++.

[tobil4sk]

Since the mutex no longer uses HxMutex, I think we should also remove the HxMutex type from everywhere else in the codebase for internal consistency.

That also officially breaks the HX_THREAD_H_OVERRIDE feature, so we would have to remove that too, or continue to have a HxMutex wrapper.

[Aidan63]

I think replacing uses of HxMutex with std::recursive_mutex makes sense. As a replacement for HX_THREAD_H_OVERRIDE maybe have a hxml & xml HX_THREAD_API_XML define which you can point to a custom xml file which will provide the implementation of the ConditionVariable and RecursiveMutex types?

[tobil4sk]

Seems reasonable, though if it is for hxml use it should be prefixed with HXCPP_ for consistency.

[Aidan63]

There's only one use of HxMutex / a bit of manual pthread condition left in Immix.cpp now, unfortunately this last one is a bit of a pain. C++ states that when the dtor of std::condition_variable gets called no thread should have it locked, however on exit hxcpp programs hang when adapting the last uses of pthread conditions to std::condition_variable as some thread seems to be holding onto the locks.