Generated with gcc -E -dM:

Vaxstation:

#define __vax__ 1
#define __vax 1
#define __unix 1
#define __unix__ 1
#define vax 1
#define __GNUC__ 2
#define unix 1

Linux:

#define __linux__ 1
#define linux 1
#define __i386__ 1
#define __i386 1
#define i386 1
#define __unix 1
#define __unix__ 1
#define __GNUC__ 2
#define __linux 1
#define unix 1

NetBSD (i386):

#define __i386__ 1 
#define __NetBSD__ 1 
#define __i386 1 
#define i386 1 
#define __unix 1 
#define __unix__ 1 
#define __GNUC__ 2 
#define unix 1 

Decstation:

#define __LANGUAGE_C 1
#define R3000 1
#define __SYSTYPE_BSD__ 1
#define SYSTYPE_BSD 1
#define __host_mips 1
#define ultrix 1
#define __MIPSEL__ 1
#define __ANSI_COMPAT 1
#define __MIPSEL 1
#define ___SYSTYPE_BSD 1
#define __mips__ 1
#define __mips 1
#define __host_mips__ 1
#define MIPSEL 1
#define __R3000__ 1
#define __ultrix__ 1
#define host_mips 1
#define __SYSTYPE_BSD 1
#define ___SYSTYPE_BSD__ 1
#define mips 1
#define __LANGUAGE_C__ 1
#define __unix 1
#define __R3000 1
#define __unix__ 1
#define __bsd4_2 1
#define __bsd4_2__ 1
#define __GNUC__ 2
#define __ultrix 1
#define LANGUAGE_C 1
#define _SYSTYPE_BSD 1
#define bsd4_2 1
#define unix 1

RS-6000:

#define ___AIX 1
#define ___IBMR2__ 1
#define _AIX 1
#define ___AIX32 1
#define __CHAR_UNSIGNED__ 1
#define ___IBMR2 1
#define ___AIX__ 1
#define ___AIX32__ 1
#define _AIX32 1
#define __GNUC__ 2
#define _IBMR2 1

Sun (Solaris):

#define __GCC_NEW_VARARGS__ 1 
#define __sparc 1 
#define __svr4__ 1 
#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 

Sun (SunOS):

#define __GCC_NEW_VARARGS__ 1 
#define __sparc 1 
#define __GNUC_MINOR__ 5 
#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 

SGI:

sgi
__EXTENSIONS__
_SVR3
mips
unix