[RFA] win32-nat.c: Simplify generation of Windows environment

[Eli Zaretskii]

> Cc: gdb-patches@sourceware.org
> From: Jim Blandy <jimb@codesourcery.com>
> Date: Thu, 07 Dec 2006 16:10:14 -0800
> 
> $ cd gdb/config
> $ grep -nH -e win32-nat */*.m? 
> i386/cygwin.mh:2:NATDEPFILES= i386-nat.o win32-nat.o corelow.o
> $ 
> 
> I think that means it's just Cygwin.

No, it just means that the native W32 build is currently not part of
the CVS.

A large portion of win32-nat.c is relevant to any native Windows
debugger.  By contrast, cygwin_internal is obviously Cygwin-specific.
So I think it would be a good practice to mark such specific portions
of code explicitly.