** Distinguishing OSes **
-MAC_OS Compiling for the `mac' window-system.
MAC_OS8 Compiling for Mac OS Classic (v8 or v9). No longer supported, all code using it can be removed.
MAC_OSX Compiling for Mac OS X. Not bare Darwin.
CYGWIN Compiling the Cygwin port.
** Distinguishing GUIs **
HAVE_NTGUI Use the native W32 GUI for windows, frames, menus&scrollbars.
+MAC_OS Compiling for the `mac' window-system.
HAVE_CARBON Compile support for the Carbon GUI. Requires MAC_OS?
+HAVE_NS Use the NeXT/OpenStep/Cocoa UI under Mac OS X or GNUstep.
+NS_IMPL_GNUSTEP Compile support for GNUSTEP implementation of NS GUI API.
+NS_IMPL_COCOA Compile support for Cocoa (Apple) implementation of NS GUI API.
HAVE_X11 Compile support for the X11 GUI.
HAVE_X_WINDOWS Compile support for X Window system
X11 ?? Makefile.in suggests it's equivalent to HAVE_X11