Fix preprocessor definitions for wxBase build under OS X.

Fix preprocessor definitions for wxBase build under OS X.

Define __WXOSX__ for non-GUI build under Darwin. Ensure that the rest of the
code compiles correctly when just __WXOSX__ is defined but neither of
__WXOSX_{CARBON,COCOA,IPHONE}__ is. This ensures that wxBase can actually be
built under Mac.

Move OS X symbols definitions in wx/platform.h after wx/setup.h inclusion as
they rely on __DARWIN__ and wxUSE_GUI values which are both define in that
file now. Still keep them before wx/chkconf.h inclusion which relies on
__WXOSX_XXX__ being defined. Yes, it's a mess and should be cleaned up more
permanently some day.

Also remove some redundancy from wx/osx/{carbon,cocoa}/private.h by factoring
out common parts into wx/osx/core/private.h. Also include this header itself
from wx/osx/private.h directly instead of including it thrice from different
sub-ports headers.

Closes #12660.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66077 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775