Make clang the default on linux.

build/standalone.gypi

 # Copyright 2012 the V8 project authors. 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
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 100 matching lines @@
         'os_posix%': 0,
       }, {
         'os_posix%': 1,
       }],
       ['(v8_target_arch=="ia32" or v8_target_arch=="x64" or v8_target_arch=="x87") and \
         (OS=="linux" or OS=="mac")', {
         'v8_enable_gdbjit%': 1,
       }, {
         'v8_enable_gdbjit%': 0,
       }],
-      ['OS=="mac"', {
+      ['OS=="linux" or OS=="mac"', {
         'clang%': 1,
       }, {
         'clang%': 0,
       }],
     ],
     # Default ARM variable settings.
     'arm_version%': 'default',
     'arm_fpu%': 'vfpv3',
     'arm_float_abi%': 'default',
     'arm_thumb': 'default',
@@ skipping 312 matching lines @@
       ],
     }],
     ['clang==1 and OS=="win"', {
       'make_global_settings': [
         # On Windows, gyp's ninja generator only looks at CC.
         ['CC', '<(make_clang_dir)/bin/clang-cl'],
       ],
     }],
   ],
 }