Mention boost_compat.h in the documentation

doc/api_ref/tls.rst

@@ -1191,7 +1191,13 @@ TLS Stream
 
 :cpp:class:`TLS::Stream` offers a Boost.Asio compatible wrapper around :cpp:class:`TLS::Client` and :cpp:class:`TLS::Server`.
 It can be used as an alternative to Boost.Asio's `ssl::stream <https://www.boost.org/doc/libs/1_66_0/doc/html/boost_asio/reference/ssl__stream.html>`_ with minor adjustments to the using code.
-It offers the following interface:
+
+To use the asio stream wrapper, a relatively recent version of boost is required.
+Include ``botan/boost_compat.h`` and check that ``BOTAN_FOUND_COMPATIBLE_BOOST_VERSION``
+is defined before including ``botan/asio_stream.h`` to be ensure compatibility at
+compile time of your application.
+
+The asio Stream offers the following interface:
 
 .. cpp:class:: template <class StreamLayer, class ChannelT> TLS::Stream
 

src/examples/tls_stream_client.cpp

@@ -1,14 +1,17 @@
 #include <iostream>
 
-#include <botan/asio_stream.h>
-#include <botan/auto_rng.h>
-#include <botan/certstor_system.h>
-#include <botan/tls.h>
+#include <botan/boost_compat.h>
+#if defined(BOTAN_FOUND_COMPATIBLE_BOOST_VERSION)
 
-#include <boost/asio.hpp>
-#include <boost/beast.hpp>
-#include <boost/bind.hpp>
-#include <utility>
+   #include <botan/asio_stream.h>
+   #include <botan/auto_rng.h>
+   #include <botan/certstor_system.h>
+   #include <botan/tls.h>
+
+   #include <boost/asio.hpp>
+   #include <boost/beast.hpp>
+   #include <boost/bind.hpp>
+   #include <utility>
 
 namespace http = boost::beast::http;
 namespace ap = boost::asio::placeholders;
@@ -113,3 +116,12 @@ int main() {
 
    return 0;
 }
+
+#else
+
+int main() {
+   std::cout << "Your boost version is too old, sorry.\n";
+   return 1;
+}
+
+#endif