First step to unbreak scons build on Mac.

Review URL: http://codereview.chromium.org/13354

git-svn-id: http://src.chromium.org/svn/trunk/src/build@6790 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

SConscript.main

@@ -565,6 +565,9 @@ mac_env.FilterOut(
         'build/SConscript.v8',
     ],
 )
+# TODO(bradnelson): this is needed for now because target_platform_mac has
+# OS_MACOSX defined in a weird way.
+mac_env.FilterOut(CPPDEFINES = ['OS_MACOSX=OS_MACOSX'])
 
 if not root_env.WantSystemLib('libevent'):
   mac_env.Append(