> cc -dumpspecs gives among lots of other info
> 
> *predefines:
> -D__ppc__ -D__NATURAL_ALIGNMENT__ -D__MACH__ -D__BIG_ENDIAN__ 
> -D__APPLE__

The predfines won't necessarily be the complete list (it's probably
sufficient though). I'm using cygwin under Windows 2000, and if I run:

    gcc -E -Wp,-dM empty.c

I get the following output:

#define _stdcall __attribute__((__stdcall__))
#define __i386__ 1
#define _X86_ 1
#define __i386 1
#define __i686 1
#define __GNUC_MINOR__ 95
#define __declspec(x) __attribute__((x))
#define pentiumpro 1
#define __CYGWIN__ 1
#define _cdecl __attribute__((__cdecl__))
#define __pentiumpro 1
#define i386 1
#define i686 1
#define __pentiumpro__ 1
#define __stdcall __attribute__((__stdcall__))
#define __unix 1
#define __unix__ 1
#define __i686__ 1
#define __GNUC__ 2
#define __cdecl __attribute__((__cdecl__))
#define __CYGWIN32__ 1
#define unix 1

If I do 

    gcc -dumpspecs

the predefines only lists:

    -D_X86_=1 -Asystem(winnt)

If I do something like:

    gcc -dumpspecs | grep -- -D

then I get a bunch more, some of which are optional (depends on what
compiler options you pass in).

Dave Hylands