To get a list of predefined macros:

    touch foo.c
    gcc -dM -E foo.c

This is described in (gcc.info)Preprocessor Options.

	(from Charles Karney)

=============================================================

orion> gcc -dM -E foo.c
       #define __GCC_NEW_VARARGS__ 1
       #define __sparc 1
       #define __svr4__ 1
       #define __GNUC_MINOR__ 7
       #define __sun 1
       #define sparc 1
       #define __sun__ 1
       #define __unix 1
       #define __unix__ 1
       #define __SVR4 1
       #define sun 1
       #define __GNUC__ 2
       #define __sparc__ 1
       #define unix 1

ppcc> gcc -dM -E foo.c
       #define __GCC_NEW_VARARGS__ 1
       #define __sparc 1
       #define __GNUC_MINOR__ 7
       #define __sun 1
       #define sparc 1
       #define __sun__ 1
       #define __unix 1
       #define __unix__ 1
       #define sun 1
       #define __GNUC__ 2
       #define __sparc__ 1
       #define unix 1