I have 3 Availability.h:
/private/var/sdk/System/Library/Frameworks/Availability.h
#ifndef __AVAILABILITY__
#define __AVAILABILITY__
#define __MAC_10_0 1000
#define __MAC_10_1 1010
#define __MAC_10_2 1020
#define __MAC_10_3 1030
#define __MAC_10_4 1040
#define __MAC_10_5 1050
#define __MAC_10_6 1060
#define __MAC_NA 9999 /* not available */
#define __IPHONE_2_0 20000
#define __IPHONE_2_1 20100
#define __IPHONE_2_2 20200
#define __IPHONE_3_0 30000
#define __IPHONE_NA 99999 /* not available */
#include <AvailabilityInternal.h>
#ifdef __IPHONE_OS_VERSION_MIN_REQUIRED
#define __OSX_AVAILABLE_STARTING(_mac, _iphone) __AVAILABILITY_INTERNAL##_iphone
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep) \
__AVAILABILITY_INTERNAL##_iphoneIntro##_DEP##_iphoneDep
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
#define __OSX_AVAILABLE_STARTING(_mac, _iphone) __AVAILABILITY_INTERNAL##_mac
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep) \
__AVAILABILITY_INTERNAL##_macIntro##_DEP##_macDep
#else
#define __OSX_AVAILABLE_STARTING(_mac, _iphone)
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep)
#endif
#endif /* __AVAILABILITY__ */
/private/var/sdk/usr/include/Availability.h
#ifndef __AVAILABILITY__
#define __AVAILABILITY__
#define __MAC_10_0 1000
#define __MAC_10_1 1010
#define __MAC_10_2 1020
#define __MAC_10_3 1030
#define __MAC_10_4 1040
#define __MAC_10_5 1050
#define __MAC_10_6 1060
#define __MAC_10_7 1070
#define __MAC_NA 9999 /* not available */
#define __IPHONE_2_0 20000
#define __IPHONE_2_1 20100
#define __IPHONE_2_2 20200
#define __IPHONE_3_0 30000
#define __IPHONE_3_1 30100
#define __IPHONE_3_2 30200
#define __IPHONE_4_0 40000
#define __IPHONE_4_1 40100
#define __IPHONE_4_2 40200
#define __IPHONE_4_3 40300
#define __IPHONE_NA 99999 /* not available */
#include <AvailabilityInternal.h>
#ifdef __IPHONE_OS_VERSION_MIN_REQUIRED
#define __OSX_AVAILABLE_STARTING(_mac, _iphone) __AVAILABILITY_INTERNAL##_iphone
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep) \
__AVAILABILITY_INTERNAL##_iphoneIntro##_DEP##_iphoneDep
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
#define __OSX_AVAILABLE_STARTING(_mac, _iphone) __AVAILABILITY_INTERNAL##_mac
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep) \
__AVAILABILITY_INTERNAL##_macIntro##_DEP##_macDep
#else
#define __OSX_AVAILABLE_STARTING(_mac, _iphone)
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep)
#endif
#endif /* __AVAILABILITY__ */
/private/var/stash/usr/include/Availability.h
#ifndef __AVAILABILITY__
#define __AVAILABILITY__
#define __MAC_10_0 1000
#define __MAC_10_1 1010
#define __MAC_10_2 1020
#define __MAC_10_3 1030
#define __MAC_10_4 1040
#define __MAC_10_5 1050
#define __MAC_10_6 1060
#define __MAC_NA 9999 /* not available */
#define __IPHONE_2_0 20000
#define __IPHONE_NA 99999 /* not available */
#include <AvailabilityInternal.h>
#ifdef __IPHONE_OS_VERSION_MIN_REQUIRED
#define __OSX_AVAILABLE_STARTING(_mac, _iphone) __AVAILABILITY_INTERNAL##_iphone
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep) \
__AVAILABILITY_INTERNAL##_iphoneIntro##_DEP##_iphoneDep
#elif defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
#define __OSX_AVAILABLE_STARTING(_mac, _iphone) __AVAILABILITY_INTERNAL##_mac
#define __OSX_AVAILABLE_BUT_DEPRECATED(_macIntro, _macDep, _iphoneIntro, _iphoneDep) \
__AVAILABILITY_INTERNAL##_macIntro##_DEP##_macDep
#endif
#endif // __AVAILABILITY__