Make use of -lppapi explicit to allow for correct library order.

native_client_sdk/src/examples/pong/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl'],
   'TARGETS': [
     {
       'NAME' : 'pong',
       'TYPE' : 'main',
       'SOURCES' : [
         'pong.cc',
         'pong.h',
         'pong_module.cc',
         'view.cc',
         'view.h',
       ],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DEST': 'examples',
   'NAME': 'pong',
   'TITLE': 'Pong',
   'DESC': """
 The Pong example demonstrates how to create a basic 2D video game and
 how to store application information in a local persistent file. This game 
 uses up and down arrow keyboard input events to move the paddle.""",
   'INFO': 'File I/O, 2D graphics, input events.'
 }