Eliminate need for build_config.h for OS_* defines.

build/SConscript.main

Index: build/SConscript.main
diff --git a/build/SConscript.main b/build/SConscript.main
index c91ab4fc9e5da87d08d5e606a6ba084b0271b5ac..e276535b867a7a95d4b2b3df5bc5ec13d64495dd 100644
--- a/build/SConscript.main
+++ b/build/SConscript.main
@@ -311,6 +311,7 @@ windows_env.Replace(
     CPPDEFINES = [
         '_UNICODE',
         'UNICODE',
+        'OS_WIN',
     ],
 
 
@@ -464,6 +465,11 @@ linux_env.Append(
 )
 
 linux_env.Replace(
+    CPPDEFINES = [
+        'OS_LINUX',
+        'OS_POSIX',
+    ],
+
     # Linking of large files uses lots of RAM, so serialize links
     # using the handy flock command from util-linux.
     LINK = 'flock $TARGET_ROOT/linker.lock ' + linux_env['LINK'],
@@ -672,6 +678,8 @@ mac_env.Append(
     ],
     CPPDEFINES = [
         'DEBUG',
+        'OS_MACOSX',
+        'OS_POSIX',
     ],
 
     FRAMEWORKPATH = [