coreaudio: shim stdatomic.h and dispatch/dispatch.h on pre-10.7 SDKs

<stdatomic.h> is C11 (GCC 4.9+, Xcode 5+) and <dispatch/dispatch.h>
requires GCD (Mac OS X 10.6+).  Neither exists on Xcode 3.1 / 10.4-
10.5 / PowerPC, so coreaudio.c fails to compile there with:

    coreaudio.c:17:23: error: stdatomic.h: No such file or directory
    coreaudio.c:20:31: error: dispatch/dispatch.h: No such file or
                              directory

On SDKs predating 10.7, provide local replacements:

  - atomic_size_t / atomic_load_explicit / atomic_fetch_add_explicit
    / atomic_fetch_sub_explicit / atomic_init -> OSAtomicAdd32Barrier
    (<libkern/OSAtomic.h>, available since 10.4).  The ring-buffer
    fill counter fits easily in int32_t (worst case ~200k samples).

  - dispatch_semaphore_t / _create / _signal / _wait / dispatch_time /
    dispatch_release -> Mach semaphore_t (<mach/semaphore.h>), which
    has been on OS X since NeXTSTEP.

The file's logic is unchanged; only the header block at the top is
conditional.  Shim macros are #undef'd at EOF so they cannot leak
into later translation units in the griffin unity build.

Modern SDKs take the existing code path unchanged.

audio/drivers/coreaudio.c

@@ -14,10 +14,92 @@
  *  If not, see <http://www.gnu.org/licenses/>.
  */
 #include <stdlib.h>
-#include <stdatomic.h>
 #include <math.h>
 
+#include <AvailabilityMacros.h>
+
+/* stdatomic.h arrived with C11 (GCC 4.9+, Xcode 5+).
+ * GCD arrived in Mac OS X 10.6 Snow Leopard.
+ * On older Apple toolchains (e.g. Xcode 3.1 / 10.4-10.5 / PowerPC),
+ * shim both to legacy OS X primitives:
+ *   - atomic_size_t / atomic_*  -> OSAtomic (libkern/OSAtomic.h, 10.4+)
+ *   - dispatch_semaphore_*      -> Mach semaphores (mach/semaphore.h) */
+#if !defined(MAC_OS_X_VERSION_10_7) || \
+    (defined(MAC_OS_X_VERSION_MIN_REQUIRED) && \
+     MAC_OS_X_VERSION_MIN_REQUIRED < MAC_OS_X_VERSION_10_7)
+#define RARCH_COREAUDIO_LEGACY_APPLE 1
+#endif
+
+#ifdef RARCH_COREAUDIO_LEGACY_APPLE
+#include <libkern/OSAtomic.h>
+#include <mach/mach.h>
+#include <mach/semaphore.h>
+#include <mach/task.h>
+#include <mach/clock_types.h>
+
+/* --- atomic shim (size_t, acquire/release subset actually used) --- */
+typedef volatile int32_t atomic_size_t;
+typedef int memory_order;
+#define memory_order_acquire 0
+#define memory_order_release 0
+#define atomic_init(ptr, val) \
+   (*(ptr) = (int32_t)(val))
+#define atomic_load_explicit(ptr, mo) \
+   ((size_t)OSAtomicAdd32Barrier(0, (ptr)))
+#define atomic_fetch_add_explicit(ptr, n, mo) \
+   ((size_t)(OSAtomicAdd32Barrier((int32_t)(n), (ptr)) - (int32_t)(n)))
+#define atomic_fetch_sub_explicit(ptr, n, mo) \
+   ((size_t)(OSAtomicAdd32Barrier(-(int32_t)(n), (ptr)) + (int32_t)(n)))
+
+/* --- GCD semaphore shim (thin wrapper over Mach semaphore_t) --- */
+#ifndef NSEC_PER_MSEC
+#define NSEC_PER_MSEC 1000000ULL
+#endif
+#ifndef NSEC_PER_SEC
+#define NSEC_PER_SEC  1000000000ULL
+#endif
+#define DISPATCH_TIME_NOW 0
+
+typedef semaphore_t dispatch_semaphore_t;
+typedef uint64_t    dispatch_time_t;
+
+static INLINE dispatch_semaphore_t dispatch_semaphore_create(long value)
+{
+   semaphore_t s;
+   if (semaphore_create(mach_task_self(), &s, SYNC_POLICY_FIFO,
+            (int)value) != KERN_SUCCESS)
+      return 0;
+   return s;
+}
+static INLINE void dispatch_semaphore_signal(dispatch_semaphore_t s)
+{
+   if (s)
+      semaphore_signal(s);
+}
+static INLINE dispatch_time_t dispatch_time(dispatch_time_t base, int64_t delta)
+{
+   (void)base;
+   return (dispatch_time_t)delta;
+}
+static INLINE long dispatch_semaphore_wait(dispatch_semaphore_t s,
+      dispatch_time_t ns)
+{
+   mach_timespec_t ts;
+   if (!s)
+      return -1;
+   ts.tv_sec  = (unsigned int)(ns / NSEC_PER_SEC);
+   ts.tv_nsec = (clock_res_t)(ns % NSEC_PER_SEC);
+   return (semaphore_timedwait(s, ts) == KERN_SUCCESS) ? 0 : 1;
+}
+static INLINE void dispatch_release(dispatch_semaphore_t s)
+{
+   if (s)
+      semaphore_destroy(mach_task_self(), s);
+}
+#else
+#include <stdatomic.h>
 #include <dispatch/dispatch.h>
+#endif
 
 #if TARGET_OS_IPHONE
 #include <AudioToolbox/AudioToolbox.h>
@@ -787,3 +869,25 @@ audio_driver_t audio_coreaudio = {
    coreaudio_buffer_size,
    coreaudio_write_raw
 };
+
+/* Undo the legacy-Apple shim macros so they don't leak into other
+ * translation units when this file is #included into griffin.c.
+ *
+ * Shim typedefs (atomic_size_t, memory_order, dispatch_semaphore_t,
+ * dispatch_time_t) are left in place: C89/C99 forbid redefining a
+ * typedef even to the same type, but nothing else in the PPC griffin
+ * bundle declares these names (task_queue.c's GCD path is gated out
+ * by HAVE_GCD; the Obj-C users are in griffin_objc.m, a separate TU).
+ * If a future change adds another GCD/atomic user in griffin.c after
+ * this point, wrap its typedefs in the same RARCH_COREAUDIO_LEGACY_APPLE
+ * gate and include the shim there too. */
+#ifdef RARCH_COREAUDIO_LEGACY_APPLE
+#undef atomic_init
+#undef atomic_load_explicit
+#undef atomic_fetch_add_explicit
+#undef atomic_fetch_sub_explicit
+#undef memory_order_acquire
+#undef memory_order_release
+#undef DISPATCH_TIME_NOW
+#undef RARCH_COREAUDIO_LEGACY_APPLE
+#endif