Here are some of the cpp macros used, together with some short explanation
of their use.  Feel free to add more macros and more categories.

** Distinguishing OSes **

MAC_OS		Compiling for some version of Mac OS?
MAC_OS8		Compiling for Mac OS version 8.  Requires MAC_OS?
MAC_OSX		Compiling for Mac OS X?  Is that also valid for Darwin?
MAC_OS_X	??  Apparently only used once in mac.c.
CYGWIN		Compiling the Cygwin port.
__CYGWIN__	Ditto
MSDOS		Compiling the MS-DOS port.
__MSDOS__	Ditto.
__DJGPP__	Major version number of the DJGPP library for the DOS port.
__DJGPP_MINOR__ Minor version number of the DJGPP library.
__GO32__	Compiling the DOS port with DJGPP v1.x (obsolete).
DOS_NT		Compiling for either the MS-DOS or native MS-Windows port.
WINDOWSNT	Compiling the native MS-Windows (W32) port.
__MINGW32__	Compiling the W32 port with the MinGW port of GCC.
_MSC_VER	Compiling the W32 port with the Microsoft C compiler.

** Distinguishing GUIs **

HAVE_NTGUI	Use the native W32 GUI for windows, frames, menus&scrollbars.
HAVE_CARBON	Compile support for the Carbon GUI.  Requires MAC_OS?
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
USE_LUCID	Use the Lucid toolkit for menus&scrollbars.  Requires HAVE_X11.
USE_MOTIF	Use the Motif toolkit for menus&scrollbars.  Requires HAVE_X11.
USE_GTK		Use the Gtk   toolkit for menus&scrollbars.  Requires HAVE_X11.

** Frame types **

FRAME_TERMCAP_P	A tty (character terminal) frame.
FRAME_X_P	A frame on X Window system.
FRAME_MSDOS_P	An MS-DOS frame (used only by the DOS port).
FRAME_MAC_P	A Mac frame.
FRAME_W32_P	A frame using native MS-Windows GUI.
FRAME_WINDOW_P	A GUI frame (like X, w32, etc.)

** Compile-time options **
REL_ALLOC	Compile in the relocatable memory allocator ralloc.c.
SYSTEM_MALLOC	Use the system library's malloc.

# arch-tag: bc80061a-1168-4911-9766-46aaf2640250