Eliminate need for build_config.h for OS_* defines.

build/build_config.h

Index: build/build_config.h
diff --git a/build/build_config.h b/build/build_config.h
index 2b9efda15a2759b5cf0617123916257178218abd..90d88205c5923cd7af009e72fe0b5f3dccfd6735 100644
--- a/build/build_config.h
+++ b/build/build_config.h
@@ -2,8 +2,10 @@
 // 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
@@ -16,11 +18,17 @@
 
 // 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
@@ -28,7 +36,9 @@
 // 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.