build: fix the case where config.gypi doesn't exist

common.gypi

@@ -10,6 +10,7 @@
   },
 
   'target_defaults': {
+    'default_configuration': 'Debug',
     'configurations': {
       'Debug': {
         'defines': [ 'DEBUG', '_DEBUG' ],

tools/gyp_addon

@@ -15,6 +15,7 @@ if __name__ == '__main__':
   config_gypi = os.path.join(node_root, 'config.gypi')
   args.extend(['-I', addon_gypi])
   args.extend(['-I', common_gypi])
+  if os.path.exists(config_gypi):
     args.extend(['-I', config_gypi])
   args.extend(['-Dlibrary=shared_library'])
   args.extend(['-Dvisibility=default'])