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 **

CYGWIN		Compiling the Cygwin port.
__CYGWIN__	Ditto
MSDOS		Compiling the MS-DOS port.
__MSDOS__	Ditto.
__DJGPP_MINOR__ Minor version number of the DJGPP library; used only in msdos.c and dosfns.c.
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.
DARWIN_OS	Compiling on Mac OS X or pure Darwin (and using s/darwin.h).

** Distinguishing GUIs **

HAVE_NTGUI	Use the native W32 GUI for windows, frames, menus&scrollbars.
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
  (It looks like, nowadays, if HAVE_X11 is set, HAVE_X_WINDOWS must
   be, and vice versa.  At least, this is true for configure, and
   msdos; not sure about nt.)
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_W32_P	A frame using native MS-Windows GUI.
FRAME_WINDOW_P	A GUI frame (like X, w32, etc.)

** Support for accessing other processes on the system **

HAVE_PROCFS     The /proc filesystem is supported.

** Compile-time options **

REL_ALLOC	Compile in the relocatable memory allocator ralloc.c.
SYSTEM_MALLOC	Use the system library's malloc.
subprocesses    System can use subprocesses (for M-x shell for example).  Defined by default, only MSDOS undefines it.
USE_LISP_UNION_TYPE Define it in lisp.h to make Lisp_Object be a union type instead of the default int.

** System specific macros, described in detail in src/s/template.h
CLASH_DETECTION
COFF
FIRST_PTY_LETTER
HAVE_PTYS
INTERRUPT_INPUT
NARROWPROTO
SEPCHAR
SYSTEM_TYPE

** Misc macros
USER_FULL_NAME   If defined, overrides the default pw->pw_gecos for getting at the full user name.  Only MSDOS overrides the default.

** Replace, the definition is trivial: SWITCH_ENUM_CAST

** Defines from src/m/*.h and src/s/*.h.  Some of these might not be used in the code anymore, so they can be removed.  The HAVE_* definitions are probably handled by autoconf, so it might be possible to just remove them from src/[sm]/*.h.


AIX
AMPERSAND_FULL_NAME
BITS_PER_EMACS_INT
BITS_PER_LONG
BITS_PER_CHAR
BITS_PER_SHORT
BITS_PER_INT
BROKEN_DATAGRAM_SOCKETS
BROKEN_FIONREAD
BROKEN_GET_CURRENT_DIR_NAME
BROKEN_NON_BLOCKING_CONNECT
BROKEN_PTY_READ_AFTER_EAGAIN
BROKEN_SA_RESTART
BROKEN_SIGAIO
BROKEN_SIGIO
BROKEN_SIGPOLL
BROKEN_SIGPTY
BSD4_2
BSD4_3
BSD_SYSTEM
CLASH_DETECTION
DATA_SEG_BITS
DATA_START
DBL_MIN_REPLACEMENT
DEFAULT_SOUND_DEVICE
DEVICE_SEP
DIRECTORY_SEP
DONT_REOPEN_PTY
DOUG_LEA_MALLOC
DebPrint
EMACSDEBUG
EMACS_CONFIGURATION
EMACS_CONFIG_OPTIONS
EMACS_INT
EMACS_UINT
FILE_SYSTEM_CASE
FLOAT_CHECK_DOMAIN
GC_LISP_OBJECT_ALIGNMENT
GC_MARK_SECONDARY_STACK
GC_MARK_STACK
GC_SETJMP_WORKS
GMALLOC_INHIBIT_VALLOC
GNU_LIBRARY_PENDING_OUTPUT_COUNT
GNU_LINUX
GNU_MALLOC
HAVE_AIX_SMT_EXP
HAVE_CBRT
HAVE_CLOSEDIR
HAVE_DUP2
HAVE_EUIDACCESS
HAVE_FMOD
HAVE_FPATHCONF
HAVE_FREXP
HAVE_FSYNC
HAVE_FTIME
HAVE_GETCWD
HAVE_GETDOMAINNAME
HAVE_GETHOSTNAME
HAVE_GETLOADAVG
HAVE_GETPAGESIZE
HAVE_GETPT
HAVE_GETTIMEOFDAY
HAVE_GETWD
HAVE_H_ERRNO
HAVE_INET_SOCKETS
HAVE_INVERSE_HYPERBOLIC
HAVE_LIBKSTAT
HAVE_LIMITS_H
HAVE_LINUX_VERSION_H
HAVE_LOGB
HAVE_LONG_FILE_NAMES
HAVE_LRAND48
HAVE_MENUS
HAVE_MKDIR
HAVE_MKTIME
HAVE_MOUSE
HAVE_PERROR
HAVE_PSTAT_GETDYNAMIC
HAVE_PWD_H
HAVE_RANDOM
HAVE_RENAME
HAVE_RES_INIT
HAVE_RINT
HAVE_RMDIR
HAVE_SELECT
HAVE_SETLOCALE
HAVE_SETPGID
HAVE_SETRLIMIT
HAVE_SETSID
HAVE_SHUTDOWN
HAVE_SOCKETS
HAVE_SOUND
HAVE_STDLIB_H
HAVE_STRERROR
HAVE_STRFTIME
HAVE_STRING_H
HAVE_STRUCT_UTIMBUF
HAVE_SYSINFO
HAVE_SYSV_SIGPAUSE
HAVE_SYS_SELECT_H
HAVE_SYS_SYSTEMINFO_H
HAVE_SYS_TIMEB_H
HAVE_SYS_TIME_H
HAVE_TCATTR
HAVE_TIMEVAL
HAVE_TM_ZONE
HAVE_TZSET
HAVE_UNISTD_H
HAVE_UTIMES
HAVE_UTIME_H
HAVE_WINDOW_SYSTEM
HAVE_WORKING_VFORK
HAVE_XRMSETDATABASE
HPUX
INTERNAL_TERMINAL
IS_ANY_SEP
IS_DIRECTORY_SEP
LINKER
LINUX_VERSION_CODE
LISP_FLOAT_TYPE
LNOFLSH
LOCALTIME_CACHE
MAIL_USE_FLOCK
MAIL_USE_LOCKF
MAIL_USE_POP
MAIL_USE_SYSTEM_LOCK
MAXPATHLEN
MIN_PTY_KERNEL_VERSION -- only used on Mac
MODE_LINE_BINARY_TEXT
MUST_UNDEF__STDC__
NLIST_STRUCT
NO_ABORT
NO_EDITRES
NO_MATHERR
NO_TERMIO
NSIG
NSIG_MINIMUM
NULL
NULL_DEVICE
ORDINARY_LINK
O_APPEND
O_RDONLY
O_RDWR
PAGESIZE
PENDING_OUTPUT_COUNT
POSIX
PREFER_VSUSP
PTY_ITERATION
PTY_NAME_SPRINTF
PTY_OPEN
PTY_TTY_NAME_SPRINTF
PURESIZE
RUN_TIME_REMAP
SA_RESTART
SEGMENT_MASK
SETPGRP_RELEASES_CTTY
SETUP_SLAVE_PTY
SIGALRM
SIGCHLD
SIGHUP
SIGKILL
SIGNALS_VIA_CHARACTERS
SIGPIPE
SIGQUIT
SIGTRAP
SOLARIS2
STDC_HEADERS
SYSTEM_PURESIZE_EXTRA
SYSTEM_MALLOC
SYSV_SYSTEM_DIR
TAB3
TABDLY
TERM
TIME_WITH_SYS_TIME
TIOCSIGSEND
TM_IN_SYS_TIME
ULIMIT_BREAK_VALUE
UNIX98_PTYS
USE_CRT_DLL
USE_TOOLKIT_SCROLL_BARS
USG
USG5
USG5_4
USG_SUBTTY_WORKS
VALBITS
WRETCODE
XOS_NEEDS_TIME_H
_AIX
_ARCH_PPC64
_FILE_OFFSET_BITS
_LP64
_MALLOC_INTERNAL
_NAIVE_DOS_REGS
_VARARGS_
_WINSOCKAPI_
_WINSOCK_H
_longjmp
_setjmp
_start
abort
access
alloca
brk
calloc
chdir
chmod
chown
close
const
creat
ctime
dup
dup2
edata
emacs
etext
execlp
execvp
fdopen
fileno
fopen
free
fsync
ftruncate
fwrite
getdefdir
getdisk
getenv
getpid
getuid
gmtime
index
isatty
kill
link
linux
localtime
logb
lseek
malloc
mkdir
mktemp
open
pclose
pipe
popen
random
read
realloc
rename
rindex
rmdir
sbrk
select
sigmask
signal
sigsetmask
sleep
spawnve
srandom
strdup
strerror
stricmp
strnicmp
strupr
sys_nerr
system
temacs
tzname
tzset
umask
unix
unlink
utimbuf
utime
vfork
wait
write
xfree