use feature test macro to check feature available

[cauliyang]

Reason

thread-pool is a great modern and tiny thread pool. However, using the compiler version to check move_only_function is not solid. For example:

I used g++-12 (Ubuntu 12.1.0-2ubuntu1~22.04) 12.1.0. The library could not compile due to:

error: ‘move_only_function’ in namespace ‘std’ does not name a template type
25 | using default_function_type = std::move_only_function<void()>;
| ^~~~~~~~~~~~~~~~~~

Here is the ci for detailed information.

Hence, I tried to use feature testing instead and it works well.

I am sure that I may have missed something. :)

[DeveloperPaul123]

Hi 👋

Thank you for the PR! I didn't realize that the feature macros were standardized in C++20, thanks for the links!

Also it seems that there was a possibly in a bug in my GNU version detection. It seems that this approach should be better but I'd like to do more testing locally.

Also nice to see others are using my thread pool 😀

[cauliyang]

Hi ✋ 

This modern C++ library is handy and powerful! I like it. We do need to conduct more testing to ensure the quality of the code.

I really appreciate your work, and I'm learning a lot!

[DeveloperPaul123]

PR looks great and works great, thanks again!

[cauliyang]

Great! 🚀 I hope this library will be best! Many thanks for your hard work!