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

native_client_sdk/src/examples/pi_generator/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl'],
   'TARGETS': [
     {
       'NAME' : 'pi_generator',
       'TYPE' : 'main',
       'SOURCES' : [
         'pi_generator.cc',
         'pi_generator.h',
         'pi_generator_module.cc'
       ],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DEST': 'examples',
   'NAME': 'pi_generator',
   'TITLE': 'Monte Carlo Estimate for Pi',
   'DESC': """
 The Pi Generator example demonstrates creating a helper thread that
 estimate pi using the Monte Carlo method while randomly putting 1,000,000,000
 points inside a 2D square that shares two sides with a quarter circle.""",
   'INFO': 'Thread creation, 2D graphics, view change events.'
 }