The .h files in Makerules/DARWIN/ contain defines that key special case handling
for the various kernels that this source base supports (currently Panther [10.3.x]
and Tiger [10.4.x]).

When adding new flags please following the following guidelines:

- Make the sense of the flags positive instead of negative so that old files
  don't need to be changed when a new kernel is added to the build
- Make the flag specific to API difference

Currently the KERNEL_VERSION flag is not used.

The following flags are currently defined:

CONFIG_PROC_FS - Enables/Disables the "/proc" implementation under Darwin
HAS_KALLOC - kalloc() is exported for use by the kernel.
HAS_CPU_NUMBER - cpu_number() is exported for use by the kernel.
HAS_KERNEL_FUNNEL - kernel funnel is exported for use by the kernel.
HAS_KERNEL_SOCKETS - kernel sockets are exported for use by the kernel.
HAS_ENABLE_PREEMPTION - preemption values are exported for getting and setting 
by the kernel.
HAS_ABSOLUTETIME - enable if clock calls use AbsoluteTime instead of uint64_t
HAS_THREAD_DEADLINE - thread_set_timer_deadline() is exported for use by the kernel.
HAS_NETWORK_LOGGING - indicates whether or not to use the CNetworkInterface class.
This is a temporary measure until the CNetworkInterface class is ported to Tiger.
HAS_PANTHER_DEBUGGING - Supports CTrace::BackTrace(), CTrace::PointerCheck()
HAS_HOSTNAME - kernel exports the _hostname symbol.
HAS_MEMORY_GLOBALS - kernel exports the _mem_actual symbol.
HAS_OPAQUE_PROCPTR - struct proc is an opaque data type, use proc_t instead.
HAS_MBUF_FUNCS - struct mbuf is opaque type, use mbuf_* functions to access
HAS_IFNET_T - struct ifnet replaced with ifnet_t
