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

native_client_sdk/src/examples/gamepad/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl'],
   'TARGETS': [
     {
       'NAME' : 'gamepad',
       'TYPE' : 'main',
       'SOURCES' : ['gamepad.cc', 'gamepad_module.cc', 'gamepad.h'],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DEST': 'examples',
   'NAME': 'gamepad',
   'TITLE': 'Gamepad Example.',
   'DESC': """
 Attached gamepad values should appear, left to right, once they've been
 interacted with. Buttons, esp triggers are analog.
 """,
   'INFO':  'Gamepad interface.'
 }