Make SDL's Objective-C build with GCC as well as clang.

GCC's Objective-C has no ARC and no @autoreleasepool, and the pre-10.15
SDKs lack the larger AudioChannelLayout tags along with the @available
keyword to guard them. Gate the CoreAudio 6.1/7.1 layouts on the SDK
version, and spell autorelease pools as SDL_OBJC_POOL (new
src/SDL_objc_compat.h): under clang it expands to @autoreleasepool
itself - with or without ARC, byte-for-byte upstream behavior - and
under GCC to an NSAutoreleasePool whose cleanup attribute drains it on
any exit from the enclosing scope, early returns included.

--- src/audio/coreaudio/SDL_coreaudio.m
+++ src/audio/coreaudio/SDL_coreaudio.m
@@ -765,6 +765,8 @@
         //  L R C LFE Ls Rs
         layout.mChannelLayoutTag = kAudioChannelLayoutTag_DVD_12;
         break;
+#if (defined(__IPHONE_OS_VERSION_MAX_ALLOWED) && __IPHONE_OS_VERSION_MAX_ALLOWED >= 130000) || \
+    (defined(MAC_OS_X_VERSION_MAX_ALLOWED) && MAC_OS_X_VERSION_MAX_ALLOWED >= 101500)
     case 7:
         if (@available(macOS 10.15, iOS 13.0, tvOS 13.0, *)) {
             // L R C LFE Cs Ls Rs
@@ -801,6 +803,7 @@
             }
         }
         break;
+#endif
     default:
         return SDL_SetError("Unsupported audio channels");
     }
--- /dev/null
+++ src/SDL_objc_compat.h
@@ -0,0 +1,52 @@
+/*
+  Simple DirectMedia Layer
+  Copyright (C) 1997-2026 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
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/* Compatibility layer that lets SDL's Objective-C sources build both with
+   clang (with or without ARC; the upstream default) and with GCC, whose
+   Objective-C has no ARC and no @autoreleasepool. Everything here compiles
+   away to the stock spelling under clang. */
+
+#ifndef SDL_objc_compat_h_
+#define SDL_objc_compat_h_
+
+#include <Foundation/Foundation.h>
+
+/* @autoreleasepool replacement. GCC cannot parse @autoreleasepool, so
+   spell pools as SDL_OBJC_POOL { ... }. The GCC version is a plain
+   declaration whose cleanup attribute drains the pool when the enclosing
+   scope exits (early return included) - slightly wider than the braces,
+   but every use is the first statement of a function body, where the two
+   are equivalent. For the same reason it can only be used once per scope,
+   and only at the top of a block. */
+#if defined(__clang__)
+#define SDL_OBJC_POOL @autoreleasepool
+#else
+static inline void SDL_ObjCDrainPool_(NSAutoreleasePool **pool)
+{
+    [*pool drain];
+}
+#define SDL_OBJC_POOL                                                   \
+    NSAutoreleasePool *sdl_objc_pool_                                   \
+        __attribute__((__cleanup__(SDL_ObjCDrainPool_), __unused__))    \
+        = [[NSAutoreleasePool alloc] init];
+#endif
+
+#endif // SDL_objc_compat_h_
--- src/filesystem/cocoa/SDL_sysfilesystem.m
+++ src/filesystem/cocoa/SDL_sysfilesystem.m
@@ -31,9 +31,11 @@
 #include <sys/stat.h>
 #include <sys/types.h>
 
+#include "SDL_objc_compat.h"
+
 char *SDL_SYS_GetBasePath(void)
 {
-    @autoreleasepool {
+    SDL_OBJC_POOL {
         NSBundle *bundle = [NSBundle mainBundle];
         const char *baseType = [[[bundle infoDictionary] objectForKey:@"SDL_FILESYSTEM_BASE_DIR_TYPE"] UTF8String];
         const char *base = NULL;
@@ -65,7 +67,7 @@
 
 char *SDL_SYS_GetExeName(void)
 {
-    @autoreleasepool {
+    SDL_OBJC_POOL {
         NSBundle *bundle = [NSBundle mainBundle];
         const char *name = [[[bundle infoDictionary] objectForKey:@"CFBundleIdentifier"] UTF8String];
         if (!name) {
@@ -87,7 +89,7 @@
 
 char *SDL_SYS_GetPrefPath(const char *org, const char *app)
 {
-    @autoreleasepool {
+    SDL_OBJC_POOL {
         char *result = NULL;
         NSArray *array;
 
@@ -143,7 +145,7 @@
 
 char *SDL_SYS_GetUserFolder(SDL_Folder folder)
 {
-    @autoreleasepool {
+    SDL_OBJC_POOL {
 #ifdef SDL_PLATFORM_TVOS
         SDL_SetError("tvOS does not have persistent storage");
         return NULL;
--- src/locale/macos/SDL_syslocale.m
+++ src/locale/macos/SDL_syslocale.m
@@ -24,9 +24,11 @@
 
 #import <Foundation/Foundation.h>
 
+#include "SDL_objc_compat.h"
+
 bool SDL_SYS_GetPreferredLocales(char *buf, size_t buflen)
 {
-    @autoreleasepool {
+    SDL_OBJC_POOL {
         NSArray *languages = NSLocale.preferredLanguages;
         size_t numlangs = 0;
         size_t i;