Fix build at 2f4232d.

build/standalone.gypi

 # Copyright 2014 PDFium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # Definitions to be used when building stand-alone PDFium binaries.
 
 {
   'variables': {
     'component%': 'static_library',
     'clang%': 0,
@@ skipping 323 matching lines @@
               # TODO(hans): Make this list shorter eventually, http://crbug.com/504657
               '-Qunused-arguments',  # http://crbug.com/504658
               '-Wno-microsoft-enum-value',  # http://crbug.com/505296
               '-Wno-unknown-pragmas',  # http://crbug.com/505314
               '-Wno-microsoft-cast',  # http://crbug.com/550065
 
               # TODO(ochang): Fix these properly eventually.
               '-Wno-inconsistent-missing-override',
             ],
           }],
-          ['clang==1 and MSVS_VERSION == "2013"', {
+          ['OS=="win" and clang==1 and MSVS_VERSION == "2013"', {
             'VCCLCompilerTool': {
               'AdditionalOptions': [
                 '-fmsc-version=1800',
               ],
             },
           }],
-          ['clang==1 and MSVS_VERSION == "2015"', {
+          ['OS=="win" and clang==1 and MSVS_VERSION == "2015"', {
             'VCCLCompilerTool': {
               'AdditionalOptions': [
                 '-fmsc-version=1900',
               ],
             },
           }],
         ],
       },
       'VCLibrarianTool': {
         'AdditionalOptions': ['/ignore:4221'],
@@ skipping 139 matching lines @@
       ],
     }],  # OS=="win"
     ["use_goma==1", {
       'make_global_settings': [
         ['CC_wrapper', '<(gomadir)/gomacc'],
         ['CXX_wrapper', '<(gomadir)/gomacc'],
       ],
     }],  # use_goma==1
   ],
 }