Convert from using env['PLATFORM'] directly to using the more flexible...

third_party/libpng/libpng.scons

 # 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.
 
 
 __doc__ = """
 Configuration for building libpng.lib / libpng.a.
 """
 
 Import('env')
 
 if env.WantSystemLib('libpng'):
   Return()
 
 env = env.Clone()
 
 env.ApplySConscript([
     '$ZLIB_DIR/using_zlib.scons',
 ])
 
 env.Prepend(
     CPPPATH = [
         '$CHROME_SRC_DIR',
     ],
 )
 
-if env['PLATFORM'] == 'win32':
+if env.Bit('windows'):
   env.Append(
       CCFLAGS = [
           '/TP',
           '/wd4800',
       ],
   )
 
 env.Append(
     CPPDEFINES = [
         'PNG_USER_CONFIG',
@@ skipping 15 matching lines @@
     'pngrio.c',
     'pngread.c',
     'pngpread.c',
     'pngmem.c',
     'pngget.c',
     'pnggccrd.c',
     'pngerror.c',
 ]
 
 env.ChromeStaticLibrary('libpng', input_files)