Add generic version of has_nothrow_assign.

Does not require a compiler intrinsic - makes it easier to port to other/unknown compilers going forward as long as they are C++11.

test/Jamfile.v2

@@ -29,6 +29,10 @@ rule all-tests {
      {
          result += [ run $(source) ] ;
      }
+     for local source in has_nothrow_assign_test
+     {
+         result += [ run $(source).cpp : : : <define>BOOST_TT_DISABLE_INTRINSICS : $(source)_no_intrinsics ] ;
+     }
      return $(result) ;     
 }