WIN32 is defined by windows.h. _WIN32 is defined by the compiler. Fix the check to use _WIN32 since it is defined 100% of the time.

Review URL: http://codereview.chromium.org/20108

git-svn-id: http://src.chromium.org/svn/trunk/src/build@9387 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

build_config.h

@@ -19,7 +19,7 @@
 #define OS_MACOSX 1
 #elif defined(__linux__)
 #define OS_LINUX 1
-#elif defined(WIN32)
+#elif defined(_WIN32)
 #define OS_WIN 1
 #else
 #error Please add support for your platform in build/build_config.h