Add support for modular build structure. (#2905)

* Use beast exclusive valgrind feature.

* Make the library modular usable.

* Switch to library requirements instead of source. As source puts extra source in install targets.

* Add Beast root dir for includes. As some tests refer to files relative to that.

* Replace relative docca refs with project based.

* Avoid mtuliple openssl configurations.

* Always declare openssl. If it's empty and duplicate it will be ignored.

* Add missing import-search for cconfig/predef checks.

* Add requires-b2 check to top-level build file.

* Bump B2 require to 5.2

* Update copyright dates.

* Move inter-lib dependencies to a project variable and into the build targets.

* Update build deps.

* Update build deps.

* The http/client/body example uses the source header from json instead of the linked library.

example/advanced/server-flex-awaitable/Jamfile

@@ -12,6 +12,7 @@ import ac ;
 project
     : requirements
         [ ac.check-library /boost/beast//lib-asio-ssl : <library>/boost/beast//lib-asio-ssl/<link>static : <build>no ]
+        <library>/boost/scope//boost_scope
     ;
 
 exe advanced-server-flex-awaitable :