Availability.h No such file or directory

I am trying to build a pre-written app, on OS开发者_如何学Go X 10.5 with the base SDK set to OS X 10.4. There are a couple of pre-compiled header files which are returning errors on compile, in CFNetwork.h where it is including Availability.h

"Availability.h - No such file or directory"

I checked the header path and this header is not present in the MacOSX10.4u.sdk /usr/include area. It is present in the corresponding 10.5 SDK location.

AvailabilityInternals.h is absent as well. But AvailabilityMacros.h is present. Obviously copying these missing headers over from 10.5 location did not work. Am I missing a package in my Leopard install that's causing this? My framework search paths seem to be ok. How to I work around this issue? Please suggest.

Thanks

http://opensource.apple.com/source/CarbonHeaders/CarbonHeaders-18.1/Availability.h

These macros are for use in OS header files. They enable function prototypes and Objective-C methods to be tagged with the OS version in which they were first available; and, if applicable, the OS version in which they became deprecated.