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

third_party/libxml/libxml.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 31 matching lines @@
         'DerivedSources',
         'DerivedSources/include',
         'include',
     ],
     CPPDEFINES = [
         'U_STATIC_IMPLEMENTATION',
         'LIBXML_STATIC',
     ],
 )
 
-if env['PLATFORM'] == 'win32':
+if env.Bit('windows'):
   env.Append(
       CCFLAGS = [
           '/TC',
           '/wd4800',
       ],
   )
-elif env['PLATFORM'] == 'posix':
+elif env.Bit('linux'):
   env.Append(
       CPPDEFINES = [
           '_REENTRANT',
       ],
   )
   if '-Wall' in env['CCFLAGS']:
     # We're not responsible for bad warning hygiene in this third party code.
     env['CCFLAGS'].remove('-Werror')
 
 
@@ skipping 39 matching lines @@
     'xmlstring.c',
     'xmlunicode.c',
     'xmlwriter.c',
     'xpath.c',
     'xpointer.c',
 ]
 
 env.ChromeStaticLibrary('libxml', input_files)
 
 
-if env['PLATFORM'] == 'win32':
+if env.Bit('windows'):
   config_files = [
       # The configure.js script must be first in this list; the
       # env.Command() call below executes the first list element.
 
       'win32/configure.js',
       'win32/Makefile.msvc',
 
       'config.h.in',
       'configure.in',
       'libxml-2.0-uninstalled.pc.in',
       'libxml-2.0.pc.in',
       'libxml.spec.in',
       'xml2-config.in',
       'xml2Conf.sh.in',
 
       'include/libxml/xmlversion.h.in',
       'include/win32config.h',
   ]
 
   copied_files = []
   for cf in config_files:
     result = env.Command('DerivedSources/' + cf, cf, Copy('$TARGET', '$SOURCE'))
     copied_files.extend(result)
 
   env.Command(['DerivedSources/config.h',
                'DerivedSources/include/libxml/xmlversion.h'],
               copied_files,
               'cd ${SOURCE.dir} && $CSCRIPT ${SOURCE.file} $CONFIG_OPTIONS',
               CONFIG_OPTIONS='compiler=msvc iconv=no icu=yes')
-elif env['PLATFORM'] == 'posix':
+elif env.Bit('linux'):
   config_files = [
       'config.h',
       'include/libxml/xmlversion.h',
       'xml2-config',
   ]
   for cf in config_files:
     result = env.Command('DerivedSources/' + cf, 'linux/' + cf,
                          Copy('$TARGET', '$SOURCE'))