Fixed boost/afio/config.hpp

I made boost/afio/config.hpp similar to other libs, particularly Boost.Atomic

I added two defines to scons to account for these changes, and edited a some of the preprocessor section of afio.hpp

This should make the setup similar to other libraries, and also builds with bjam w/ link=shared

compiles under gcc 4.7
Replaced all instances of BOOST_AFIO_API with BOOST_AFIO_DECL to match other libs