Log Message:
Re-work how we handle MacTypes.h.  Older Darwin releases do not ship a copy in
/usr/include, whilst newer Xcode do not ship a copy in the SDK, and we somehow
need to handle both.  This package explicitly passes -mmacosx-version-min=10.4
meaning we cannot test the __ENVIRONMENT_MAC_OS_X_VERSION_MIN_REQUIRED__ macro
as we would normally.  So for now we perform a direct substitution based on
whether /usr/include/MacTypes.h exists.  Cleaner solutions welcome.