Patch required to build SDL against the latest ppapi headers

Patch required to build SDL against the latest ppapi  headers.

This works around the problem that more recent
ppapi version have moved the declaration of CallbackFactories 
form:
ppapi/cpp/completion_callback.h
to the new file:
ppapi/utility/completion_callback_factory.h

This causes quite a bit of headache in nacl-ports
as we build it with a large variety of SDKs which
all have different sets of ppapi headers.

For now we (ab)use a versioned symbol  from pp_var to control the
the include behavior.

This could be made somewhat cleaner if ppapi exported 
a number symbol based on the say the svn rev, e.g.
#define PPPAI_VERSION_CONTROL_NO 9123456

which could be use in nacl-ports lik so:
#if  PPPAI_VERSION_CONTROL_NO > x



BUG= http://code.google.com/p/naclports/issues/detail?id=27
Committed: https://code.google.com/p/naclports/source/detail?r=519

[robertm, 2012-01-20 17:56:14]

PTAL

[bradn, 2012-01-20 19:05:51]

lgtm