Issue 5433 - add option to use prebuilt v8

libadblockplus.gyp

 {
   'conditions': [[
     # We don't want to use curl on Windows and Android, skip the check there
     'OS=="win" or OS=="android"',
     {
       'variables': {
         'have_curl': 0
       }
     },
     {
@@ skipping 50 matching lines @@
           }
         }
       ]],
     },
     'conditions': [
       ['OS=="linux" or OS=="mac"', {
         'link_settings': {
           'libraries': [
             '<@(libv8_libs)'
           ],
+          'library_dirs': [
+            '<(libv8_lib_dir)'
+          ]
         }
       }],
       ['OS=="win"', {
         'link_settings': {
           'libraries': [
             '<@(libv8_libs)',
             '-lwinmm'
           ],
         },
       }],
@@ skipping 118 matching lines @@
       'test/WebRequest.cpp'
     ],
     'msvs_settings': {
       'VCLinkerTool': {
         'SubSystem': '1',   # Console
         'EntryPointSymbol': 'mainCRTStartup',
       },
     },
   }]
 }