Define and use the CAMLthread_local macro for TLS variables

[MisterDA]

The current way of declaring Thread-Local Storage for variables, __thread, isn't available under MSVC.

To declare a variable in Thread-Local Storage (TLS):

• GNU C compatible compilers have the __thread storage class keyword;
• MSVC has the __declspec(thread) extended storage-class modifier;
• C11 has the _Thread_local storage-class specifier, and the thread_local macro defined in the (optionally provided) <threads.h> header;
• C23 has the thread_local storage-class specifier as a keyword, and removes _Thread_local;
• C++11 has the thread_local storage-class specifier as a keyword;
• In C11/C17, if implementations don't provide <threads.h>, they must define __STDC_NO_THREADS__. Neither MinGW nor MSVC provide the header or define the macro (non-compliant compilers…).

Adding #define __thread __declspec(thread) for MSVC wouldn't be standard-compliant, as identifiers beginning with an underscore are reserved.

Although it would be nice to use thread_local everywhere, if we use it in a public header we risk it may conflict with a previous declaration from a user. That's unlikely, because users not having a sane definition of thread_local as a macro would expose themselves to the same compatibility problems, and their code will start breaking in C23 when it becomes a keyword.
If we had private headers we could also have used thread_local internally, and have a compatibility definition for our public headers.

We take the disruptive approach of using standard keywords instead of compilers extensions, and the conservative approach of hiding them below the new CAMLthread_local macro.

In the caml/misc.h header, we assume that the file is being compiled at least in C11 or C++11. We already use the _Atomic keyword in public headers, introduced in C11. C++11 is also required for atomics.

[xavierleroy]

LGTM. It makes a lot of sense to hide the actual mechanism behind a CAMLthread_local macro, and the proposed definition of the macro looks good to me. MSVC may need a special case later, but that's business as usual.