Include AvailabilityMacros.h only where it is used.

Source/wtf/FastMalloc.cpp

 // Copyright (c) 2005, 2007, Google Inc.
 // All rights reserved.
 // Copyright (C) 2005, 2006, 2007, 2008, 2009, 2011 Apple Inc. All rights reserved.
 // 
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 // 
 //     * Redistributions of source code must retain the above copyright
 // notice, this list of conditions and the following disclaimer.
@@ skipping 64 matching lines @@
 //   goes from about 1100 ns to about 300 ns.
 
 #include "config.h"
 #include "wtf/FastMalloc.h"
 
 #include "wtf/Assertions.h"
 #include "wtf/CPU.h"
 #include "wtf/StdLibExtras.h"
 #include "wtf/UnusedParam.h"
 
+#if OS(DARWIN)
+#include <AvailabilityMacros.h>
+#endif
+
 #include <limits>
 #if OS(WINDOWS)
 #include <windows.h>
 #else
 #include <pthread.h>
 #endif
 #include <string.h>
 
 #ifndef NO_TCMALLOC_SAMPLES
 #define NO_TCMALLOC_SAMPLES
@@ skipping 4061 matching lines @@
 void FastMallocZone::init()
 {
     static FastMallocZone zone(pageheap, &thread_heaps, static_cast<TCMalloc_Central_FreeListPadded*>(central_cache), &span_allocator, &threadheap_allocator);
 }
 
 #endif // OS(DARWIN)
 
 } // namespace WTF
 
 #endif // FORCE_SYSTEM_MALLOC