Fix unused parameter warnings and missing includes:

fix #127

* Added beast::detail::ignore_unused based on boost::ignore_unused
* Added -Wextra compilation flag when building with gcc
* Fixed all unused parameter warnings with ignore_unused
* Fixed all missing includes when building each .hpp separately

include/beast/http/impl/message.ipp

@@ -12,6 +12,7 @@
 #include <beast/http/concepts.hpp>
 #include <beast/http/rfc7230.hpp>
 #include <beast/core/detail/ci_char_traits.hpp>
+#include <beast/core/detail/type_traits.hpp>
 #include <boost/assert.hpp>
 #include <boost/optional.hpp>
 #include <stdexcept>
@@ -61,6 +62,7 @@ void
 prepare_options(prepare_info& pi,
     message<isRequest, Body, Headers>& msg)
 {
+    beast::detail::ignore_unused(pi, msg);
 }
 
 template<bool isRequest, class Body, class Headers>
@@ -69,6 +71,7 @@ prepare_option(prepare_info& pi,
     message<isRequest, Body, Headers>& msg,
         connection value)
 {
+    beast::detail::ignore_unused(msg);
     pi.connection_value = value;
 }
 
@@ -106,6 +109,7 @@ prepare_content_length(prepare_info& pi,
     message<isRequest, Body, Headers> const& msg,
         std::false_type)
 {
+    beast::detail::ignore_unused(msg);
     pi.content_length = boost::none;
 }