Introduce <botan/boost_compat.h> as feature flag

[reneme]

This introduces #include <botan/boost_compat.h> that defines BOTAN_MINIMUM_SUPPORTED_BOOST_VERSION as the minimum version of boost to be usable by this version of Botan. If the available boost headers are new enough, BOTAN_FOUND_COMPATIBLE_BOOST_VERSION is defined as well. In this case, it is safe to #include <botan/asio_stream.h>, otherwise a macro-error would be generated.

Usage example:

#include <botan/boost_compat.h>

#if defined(BOTAN_FOUND_COMPATIBLE_BOOST_VERSION)
#include <botan/asio_stream.h>
// profit...
#else
// fallback...
#endif

[coveralls]

coverage: 91.718%. remained the same when pulling 8eaa184 on fix/asio_compat into 0981814 on master.

[randombit]

Left a couple of minor comments (and I concur on the comments @lieser left) but basically LGTM.

Can you please amend the relevant docs/examples?

[randombit]

This comment should be clarified that it only applies to the asio integrations; we can still use older Boost for eg socket wrappers.

[reneme]

I applied the suggestions, extended src/examples/tls_stream_client.cpp and added a note about BOTAN_FOUND_COMPATIBLE_BOOST_VERSION in the documentation.