Handle C++ libraries without threading support.

[xloem]

Adds preprocessor checks for __STDCPP_THREADS__ before making use of std::thread.

Standard definition of define: https://timsong-cpp.github.io/cppwp/n3337/cpp.predefined#2.6
Example of how header files function if it is not present: https://github.com/llvm-mirror/libcxx/blob/78d6a7767ed57b50122a161b91f59f19c9bd0d19/include/thread#L109

Test failures were because older versions of gcc didn't have __STDCPP_THREADS__ yet.

• added check for _GLIBCXX_HAS_GTHREADS for older gcc's
• added chrono for perplexity example, which was previously coming in from one of the <thread> includes

[evanmiller]

@howard0su I see you have done some threading work, what do you think of this change?

[ggerganov]

I don't think it is worth supporting this as the application is very niche.
The change is rather simple for anyone how needs it to custom implement it in their project

[xloem]

To clarify, my usecase was wasm.