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.
+** 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.