Make RefCounter compile on OS X with the 10.4 SDK

Make RefCounter compile on OS X with the 10.4 SDK

The 10.4 SDK defines OSAtomic functions as int32_t, while 10.5 and newer SDKs
define them as volatile int32_t. This caused a compilation error when
compiling against the 10.4 SDK.

I'd have prefered a preprocessor-based solution, but I couldn't find any
macro that says the SDK version, so I copied this cast solution from
Apple's WebKit. I assume then know what they are doing if they have to
workaround their own API. :)