Eliminate need for build_config.h for OS_* defines.

build/build_config.h

 // Copyright (c) 2006-2008 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-// This file adds defines about the platform we're currently building on.
-//  Operating System:
+// This file adds and checks for defines about the platform we're currently
+// building on.  (The OS_* defines were historically set here, but now they're
+// part of the build configuration and this file just asserts that they're set.)
+//  Operating System (set by build configuration):
 //    OS_WIN / OS_MACOSX / OS_LINUX / OS_POSIX (MACOSX or LINUX)
 //  Compiler:
 //    COMPILER_MSVC / COMPILER_GCC
 //  Processor:
 //    ARCH_CPU_X86 / ARCH_CPU_X86_64 / ARCH_CPU_X86_FAMILY (X86 or X86_64)
 //    ARCH_CPU_32_BITS / ARCH_CPU_64_BITS
 
 #ifndef BUILD_BUILD_CONFIG_H_
 #define BUILD_BUILD_CONFIG_H_
 
 // A set of macros to use for platform detection.
 #if defined(__APPLE__)
-#define OS_MACOSX 1
+#ifndef OS_MACOSX 1
+#error OS_MACOSX should have been set by build files.
+#endif
 #elif defined(__linux__)
-#define OS_LINUX 1
+#ifndef OS_LINUX
+#error OS_LINUX should have been set by build files.
+#endif
 #elif defined(_WIN32)
-#define OS_WIN 1
+#ifndef OS_WIN
+#error OS_WIN should have been set by build files.
+#endif
 #else
 #error Please add support for your platform in build/build_config.h
 #endif
 
 // For access to standard POSIX features, use OS_POSIX instead of a more
 // specific macro.
 #if defined(OS_MACOSX) || defined(OS_LINUX)
-#define OS_POSIX 1
+#ifndef OS_POSIX
+#error OS_POSIX should have been set by build files.
+#endif
 #endif
 
 // Compiler detection.
 #if defined(__GNUC__)
 #define COMPILER_GCC 1
 #elif defined(_MSC_VER)
 #define COMPILER_MSVC 1
 #else
 #error Please add support for your compiler in build/build_config.h
 #endif
@@ skipping 18 matching lines @@
 #define WCHAR_T_IS_UTF16
 #elif defined(OS_POSIX) && defined(COMPILER_GCC) && \
     defined(__WCHAR_MAX__) && __WCHAR_MAX__ == 0x7fffffff
 #define WCHAR_T_IS_UTF32
 #else
 #error Please add support for your compiler in build/build_config.h
 #endif
 
 #endif  // BUILD_BUILD_CONFIG_H_