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

native_client_sdk/src/examples/multithreaded_input_events/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl'],
   'TARGETS': [
     {
       'NAME' : 'mt_input_events',
       'TYPE' : 'main',
       'SOURCES' : [
         'custom_events.cc',
         'custom_events.h',
         'mt_input_events.cc',
         'shared_queue.h',
       ],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DEST': 'examples',
   'NAME': 'mt_input_events',
   'TITLE': 'Multi-threaded Input Events',
   'DESC': """
 The Multithreaded Input Events example combines HTML, Javascript,
 and C++ (the C++ is compiled to create a .nexe file).
 The C++ shows how to handle input events in a multi-threaded application.
 The main thread converts input events to non-pepper events and puts them on
 a queue. The worker thread pulls them off of the queue, converts them to a
 string, and then uses CallOnMainThread so that PostMessage can be send the
 result of the worker thread to the browser.""",
   'INFO': 'Multithreaded event handling.'
 }