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

native_client_sdk/src/examples/hello_world/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl', 'win'],
   'TARGETS': [
     {
       'NAME' : 'hello_world',
       'TYPE' : 'main',
       'SOURCES' : ['hello_world.c'],
+      'LIBS': ['ppapi', 'pthread']
     }
   ],
   'DEST': 'examples',
   'NAME': 'hello_world',
   'TITLE': 'Hello World.',
   'DESC': """
 The Hello World In C example demonstrates the basic structure of all
 Native Client applications. This example loads a Native Client module.  The
 page tracks the status of the module as it load.  On a successful load, the
 module will post a message containing the string "Hello World" back to
 JavaScript which will display it as an alert.""",
   'INFO': 'Basic HTML, JavaScript, and module architecture.'
 }