Fix atomic ops on iOS

Patch from John Ralls.

src/hb-atomic-private.hh

@@ -69,15 +69,17 @@ typedef long hb_atomic_int_t;
 #elif !defined(HB_NO_MT) && defined(__APPLE__)
 
 #include <libkern/OSAtomic.h>
+#ifdef __MAC_OS_X_MIN_REQUIRED
 #include <AvailabilityMacros.h>
+#elif defined(__IPHONE_OS_MIN_REQUIRED)
+#include <Availability.h>
+#endif
 
 typedef int32_t hb_atomic_int_t;
 #define hb_atomic_int_add(AI, V)	(OSAtomicAdd32Barrier ((V), &(AI)) - (V))
 
 #define hb_atomic_ptr_get(P)		(OSMemoryBarrier (), (void *) *(P))
-#if (MAX_OS_X_VERSION_MAX_ALLOWED >= MAX_OS_X_VERSION_10_5)
-/* XXX We should expand the above check to also allow iPhone OS >= 2.0, but I can't find any info re
- * AvailabilityMacros.h for iOS.  Is it even available there? */
+#if (MAC_OS_X_VERSION_MIN_REQUIRED > MAC_OS_X_VERSION_10_4 || __IPHONE_VERSION_MIN_REQUIRED >= 20100)
 #define hb_atomic_ptr_cmpexch(P,O,N)	OSAtomicCompareAndSwapPtrBarrier ((void *) (O), (void *) (N), (void **) (P))
 #else
 #if __ppc64__ || __x86_64__