rtems.h

gcc-you can use this code to learn something about gcc, and inquire further into linux,

/* Definitions for rtems targeting a PowerPC using elf.
   Copyright (C) 1996, 1997, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
   Contributed by Joel Sherrill ([e-mail]).

This file is part of GNU CC.

GNU CC is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2, or (at your option)
any later version.

GNU CC is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU General Public License for more details.

You should have received a copy of the GNU General Public License
along with GNU CC; see the file COPYING.  If not, write to
the Free Software Foundation, 59 Temple Place - Suite 330,
Boston, MA 02111-1307, USA.  */

/* Specify predefined symbols in preprocessor.  */

#undef TARGET_OS_CPP_BUILTINS
#define TARGET_OS_CPP_BUILTINS()          \
  do                                      \
    {                                     \
      builtin_define_std ("PPC");         \
      builtin_define ("__rtems__");       \
      builtin_assert ("system=rtems");    \
      builtin_assert ("cpu=powerpc");     \
      builtin_assert ("machine=powerpc"); \
    }                                     \
  while (0)

#undef CPP_OS_DEFAULT_SPEC
#define CPP_OS_DEFAULT_SPEC "%(cpp_os_rtems)"