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

Author: grafikrobot

Jamfile.v2

@@ -3,6 +3,8 @@
 # Distributed under the Boost Software License, Version 1.0.
 # (See accompanying file LICENSE_1_0.txt or http://www.boost.org/LICENSE_1_0.txt)
 
+require-b2 5.1 ;
+
 import option ;
 import path ;