Some updates to POSIX macros
First attempt to resolve conflict with BOOST_CHECK THROW

Using http://svn.boost.org/svn/boost/trunk/libs/multiprecision/test/test.hpp as an example I have tried to define BOOST_AFIO_CHECK_THROWS(x) to check that expression x will indeed throw an exception, else fail via BOOST_FAIL(message).

This compiles on GCC 4.7