Issue 5433 - add option to use prebuilt v8

v8.gypi

 {
   'variables': {
-    'libv8_include_dir': 'third_party/v8/include',
+    'libv8_include_dir%': 'third_party/v8/include',
     'libv8_build_targets': [],
+    'libv8_no_build%': 0,
     'conditions': [[
       'OS=="win"', {
-        'libv8_lib_dir': 'v8/build/<(CONFIGURATION_NAME)',
+        'libv8_lib_dir%': 'v8/build/<(CONFIGURATION_NAME)',
         'libv8_libs': [
           '-lv8_libplatform',
           '-lv8_base_0',
           '-lv8_base_1',
           '-lv8_base_2',
           '-lv8_base_3',
           '-lv8_libbase',
           '-lv8_libsampler',
           '-lv8_snapshot',
         ],
       }],
       ['OS=="linux" or OS=="mac"', {
+        'libv8_lib_dir%': 'v8/out/<(CONFIGURATION_NAME)',
         'libv8_libs': [
-          'v8/out/<(CONFIGURATION_NAME)/libv8_libplatform.a',
-          'v8/out/<(CONFIGURATION_NAME)/libv8_base.a',
-          'v8/out/<(CONFIGURATION_NAME)/libv8_snapshot.a',
-          'v8/out/<(CONFIGURATION_NAME)/libv8_libbase.a',
-          'v8/out/<(CONFIGURATION_NAME)/libv8_libsampler.a',
+          '-lv8_libplatform',
+          '-lv8_base',
+          '-lv8_snapshot',
+          '-lv8_libbase',
+          '-lv8_libsampler',
         ]
       }],
       ['OS=="android"', {
+        'libv8_lib_dir%': '', # must be specified, e.g. in Makefile
         'libv8_libs': [
-          'android_<(target_arch).release/libv8_libplatform.a',
-          'android_<(target_arch).release/libv8_base.a',
-          'android_<(target_arch).release/libv8_snapshot.a',
-          'android_<(target_arch).release/libv8_libbase.a',
-          'android_<(target_arch).release/libv8_libsampler.a',
+          '<(libv8_lib_dir)/libv8_libplatform.a',

[sergei, 2017/07/20 12:09:05]

For android it's required to specify .a files, one cannot simply use
-lsome_thing.

+          '<(libv8_lib_dir)/libv8_base.a',
+          '<(libv8_lib_dir)/libv8_snapshot.a',
+          '<(libv8_lib_dir)/libv8_libbase.a',
+          '<(libv8_lib_dir)/libv8_libsampler.a',
         ],
       }],
     ]
   },
   'conditions': [[
-    'OS=="win"', {
+    'OS=="win" and libv8_no_build!="true"', {
       'variables': {
         'libv8_build_targets': ['build-v8'],
       },
       'targets': [{
         'target_name': 'build-v8',
         'type': 'none',
         'actions': [{
           'action_name': 'build-v8',
           'inputs': ['build-v8.cmd'],
           'outputs': [
@@ skipping 13 matching lines @@
             '$(MSBuildBinPath)',
             '<(target_arch)',
             '<(CONFIGURATION_NAME)',
             '$(PlatformToolset)'
           ]
         }],
       }]
     }
   ]],
 }