Remove defined(__WXMAC__) && defined(__APPLE__) case from the OS if/elif block.

Remove defined(__WXMAC__) && defined(__APPLE__) case from the OS if/elif block.
It would never have been used in a configure build since setup.h which defines
__UNIX__ is included from platform.h before the if/elif block.  Likewise, it
would not have been used for wxMac Xcode builds since they use config_xcode.h
which also defines __UNIX__.  That's fortunate since __POWERPC__ was defined
unconditionally in that case.  Note that classic builds wouldn't have used
the case either since they have their own case just before it (which remains).

Add __MACH__ to the list of UNIX defines (e.g. like __hpux) so that the UNIX
case of the OS if/elif block is guaranteed to match for all Mach systems.  In
addition to that, within that case check for __APPLE__ && __MACH__ and make
sure that __BSD__ and __DARWIN__ are defined, TARGET_CARBON is defined,
and wxSIZE_T_IS_ULONG is defined.

Add __ppc__ to the list of defines that will cause __POWERPC__ to be defined
although it's interesting to note that we don't seem to use it anywhere.

Make sure that each case of the OS if/elif block has a /* OS ... */ comment
to make the code a bit easier to follow.

Remove comment about __DARWIN__ being used to work around defects in GCC,
that was removed years ago.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@49408 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775