Make all include paths relative to a root directory:

* Better include path support in the VSProject scons tool.
* Various manual fixes to include paths.

beast/asio/tests/enable_wait_for_async.test.cpp

@@ -18,13 +18,13 @@
 //==============================================================================
 
 #if BEAST_INCLUDE_BEASTCONFIG
-#include "../../../BeastConfig.h"
+#include <BeastConfig.h>
 #endif
 
-#include "../../unit_test/suite.h"
+#include <beast/unit_test/suite.h>
 
-#include "../bind_handler.h"
-#include "../enable_wait_for_async.h"
+#include <beast/asio/bind_handler.h>
+#include <beast/asio/enable_wait_for_async.h>
 
 #include <boost/asio/io_service.hpp>