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

native_client_sdk/src/examples/hello_world_interactive/example.dsc

 {
   'TOOLS': ['newlib', 'glibc', 'pnacl', 'win'],
   'TARGETS': [
     {
       'NAME' : 'hello_world_interactive',
       'TYPE' : 'main',
       'SOURCES' : [
         'hello_world.cc',
         'helper_functions.cc',
         'helper_functions.h'
       ],
-      'LIBS': ['ppapi_cpp']
+      'LIBS': ['ppapi_cpp', 'ppapi', 'pthread']
     }
   ],
   'DEST': 'examples',
   'NAME': 'hello_world_interactive',
   'TITLE': 'Interactive Hello World in C++',
   'DESC': """
 The Interactive Hello World C++ example demonstrates the basic structure 
 of all Native Client applications. This example loads a Native Client module
 which uses two way interaction with JavaScript whenever a button is clicked.
 The NaCl module will respond with the number 42 or the reversed version of the
 string in the text box when the appropriate button is clicked.""",
   'INFO': 'Basic HTML, JavaScript, C++ PPAPI, and Messaging API.',
 }