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

native_client_sdk/src/examples/load_progress/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl', 'win'],
   'TARGETS': [
     {
       'NAME' : 'load_progress',
       'TYPE' : 'main',
       'SOURCES' : ['load_progress.cc'],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DATA': ['check_browser.js'],
   'DEST': 'examples',
   'NAME': 'load_progress',
   'TITLE': 'Load Progress',
   'DESC': """
 The Load Progress example demonstrates how to listen for and handle 
 events that occur while a NaCl module loads.  This example listens for
 different load event types and dispatches different events to their
 respective handler. This example also checks for valid browser version and
 shows how to calculate and display loading progress.""",
   'INFO': 'Progress event handling.'
 }