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

third_party/libjpeg/libjpeg.scons

 # Copyright 2008, Google Inc.
 # All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #    * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #    * Redistributions in binary form must reproduce the above
@@ skipping 23 matching lines @@
 
 if env.WantSystemLib('libjpeg'):
   Return()
 
 env.Prepend(
     CPPPATH = [
         '$CHROME_SRC_DIR',
     ],
 )
 
-if env['PLATFORM'] == 'win32':
+if env.Bit('windows'):
   env.Append(
       CCFLAGS = [
           '/TC',
           '/wd4800',
       ],
   )
-elif env['PLATFORM'] == 'posix':
+elif env.Bit('linux'):
   if '-Wall' in env['CCFLAGS']:
     # We're not responsible for bad warning hygiene in this third party code.
     env['CCFLAGS'].remove('-Wall')
 
 input_files = [
     'jcapimin.c',
     'jcapistd.c',
     'jccoefct.c',
     'jccolor.c',
     'jcdctmgr.c',
@@ skipping 31 matching lines @@
     'jidctfst.c',
     'jidctint.c',
     'jmemmgr.c',
     'jmemnobs.c',
     'jquant1.c',
     'jquant2.c',
     'jutils.c',
 ]
 
 env.ChromeStaticLibrary('libjpeg', input_files)