Update to SDL 2.24.0 and add separate macOS UB2 dylib

Update mingw and macOS SDL libraries to 2.24.0. (MSVC .lib files and
Xcode project haven't been updated.)

There is now a separate SDL dylib for macOS Universal Binary 1 and 2
because SDL 2.24.0 dropped support for macOS 10.6 (which is last to
support x86). Documentation says SDL 2.24.0 also dropped macOS 10.7
and 10.8 too. So UB2 is now 10.9 or later while 10.5 to 10.8 live in
UB1.

Compiled using osxcross with macOS 11.3 SDK. Ran "strip -x" on Windows
and macOS libraries. Ad-hoc signed dylib using PyOxidizer rcodesign to
fix digest checksums after strip.

code/SDL2/include/SDL_thread.h

@@ -1,6 +1,6 @@
 /*
   Simple DirectMedia Layer
-  Copyright (C) 1997-2021 Sam Lantinga <slouken@libsdl.org>
+  Copyright (C) 1997-2022 Sam Lantinga <slouken@libsdl.org>
 
   This software is provided 'as-is', without any express or implied
   warranty.  In no event will the authors be held liable for any damages
@@ -35,7 +35,7 @@
 #include "SDL_atomic.h"
 #include "SDL_mutex.h"
 
-#if defined(__WIN32__)
+#if defined(__WIN32__) || defined(__GDK__)
 #include <process.h> /* _beginthreadex() and _endthreadex() */
 #endif
 #if defined(__OS2__) /* for _beginthread() and _endthread() */
@@ -88,7 +88,7 @@ typedef enum {
 typedef int (SDLCALL * SDL_ThreadFunction) (void *data);
 
 
-#if defined(__WIN32__)
+#if defined(__WIN32__) || defined(__GDK__)
 /**
  *  \file SDL_thread.h
  *
@@ -123,24 +123,18 @@ typedef void (__cdecl * pfnSDL_CurrentEndThread) (unsigned code);
 #define SDL_endthread _endthreadex
 #endif
 
-/**
- *  Create a thread.
- */
 extern DECLSPEC SDL_Thread *SDLCALL
 SDL_CreateThread(SDL_ThreadFunction fn, const char *name, void *data,
                  pfnSDL_CurrentBeginThread pfnBeginThread,
                  pfnSDL_CurrentEndThread pfnEndThread);
 
 extern DECLSPEC SDL_Thread *SDLCALL
-SDL_CreateThreadWithStackSize(int (SDLCALL * fn) (void *),
+SDL_CreateThreadWithStackSize(SDL_ThreadFunction fn,
                  const char *name, const size_t stacksize, void *data,
                  pfnSDL_CurrentBeginThread pfnBeginThread,
                  pfnSDL_CurrentEndThread pfnEndThread);
 
 
-/**
- *  Create a thread.
- */
 #if defined(SDL_CreateThread) && SDL_DYNAMIC_API
 #undef SDL_CreateThread
 #define SDL_CreateThread(fn, name, data) SDL_CreateThread_REAL(fn, name, data, (pfnSDL_CurrentBeginThread)SDL_beginthread, (pfnSDL_CurrentEndThread)SDL_endthread)
@@ -205,6 +199,8 @@ SDL_CreateThreadWithStackSize(SDL_ThreadFunction fn, const char *name, const siz
  *          new thread could not be created; call SDL_GetError() for more
  *          information.
  *
+ * \since This function is available since SDL 2.0.0.
+ *
  * \sa SDL_CreateThreadWithStackSize
  * \sa SDL_WaitThread
  */
@@ -250,6 +246,8 @@ SDL_CreateThread(SDL_ThreadFunction fn, const char *name, void *data);
  *          new thread could not be created; call SDL_GetError() for more
  *          information.
  *
+ * \since This function is available since SDL 2.0.9.
+ *
  * \sa SDL_WaitThread
  */
 extern DECLSPEC SDL_Thread *SDLCALL
@@ -267,6 +265,8 @@ SDL_CreateThreadWithStackSize(SDL_ThreadFunction fn, const char *name, const siz
  * \returns a pointer to a UTF-8 string that names the specified thread, or
  *          NULL if it doesn't have a name.
  *
+ * \since This function is available since SDL 2.0.0.
+ *
  * \sa SDL_CreateThread
  */
 extern DECLSPEC const char *SDLCALL SDL_GetThreadName(SDL_Thread *thread);
@@ -283,6 +283,8 @@ extern DECLSPEC const char *SDLCALL SDL_GetThreadName(SDL_Thread *thread);
  *
  * \returns the ID of the current thread.
  *
+ * \since This function is available since SDL 2.0.0.
+ *
  * \sa SDL_GetThreadID
  */
 extern DECLSPEC SDL_threadID SDLCALL SDL_ThreadID(void);
@@ -298,6 +300,8 @@ extern DECLSPEC SDL_threadID SDLCALL SDL_ThreadID(void);
  * \returns the ID of the specified thread, or the ID of the current thread if
  *          `thread` is NULL.
  *
+ * \since This function is available since SDL 2.0.0.
+ *
  * \sa SDL_ThreadID
  */
 extern DECLSPEC SDL_threadID SDLCALL SDL_GetThreadID(SDL_Thread * thread);
@@ -312,6 +316,8 @@ extern DECLSPEC SDL_threadID SDLCALL SDL_GetThreadID(SDL_Thread * thread);
  * \param priority the SDL_ThreadPriority to set
  * \returns 0 on success or a negative error code on failure; call
  *          SDL_GetError() for more information.
+ *
+ * \since This function is available since SDL 2.0.0.
  */
 extern DECLSPEC int SDLCALL SDL_SetThreadPriority(SDL_ThreadPriority priority);
 
@@ -343,6 +349,8 @@ extern DECLSPEC int SDLCALL SDL_SetThreadPriority(SDL_ThreadPriority priority);
  *               from the thread function by its 'return', or NULL to not
  *               receive such value back.
  *
+ * \since This function is available since SDL 2.0.0.
+ *
  * \sa SDL_CreateThread
  * \sa SDL_DetachThread
  */
@@ -440,6 +448,8 @@ extern DECLSPEC int SDLCALL SDL_TLSSet(SDL_TLSID id, const void *value, void (SD
 
 /**
  * Cleanup all TLS data for this thread.
+ *
+ * \since This function is available since SDL 2.0.16.
  */
 extern DECLSPEC void SDLCALL SDL_TLSCleanup(void);