Implement BOOST_OVERRIDE

doc/macro_reference.qbk

@@ -685,6 +685,8 @@ compilers implementing an early draft of the C++11 standard (in particular, inco
 [[`BOOST_NO_CXX11_NUMERIC_LIMITS`][The standard library `<limits>` header does
 not support the C++11 version of `numeric_limits`. 
 ]]
+[[`BOOST_NO_CXX11_OVERRIDE`][The compiler does not support `override`.
+]]
 [[`BOOST_NO_CXX11_POINTER_TRAITS`][The standard library does not provide a
 C++11 version of `std::pointer_traits` in <memory>.]]
 [[`BOOST_NO_CXX11_RANGE_BASED_FOR`][The compiler does not support
@@ -793,6 +795,10 @@ with:
   static BOOST_CONSTEXPR_OR_CONST UIntType xor_mask = a;
 ``
 ]]
+[[`BOOST_OVERRIDE`][
+If `BOOST_NO_CXX11_OVERRIDE` is not defined (i.e. C++11 compliant compilers),
+expands to `override` keyword, otherwise expands to nothing.
+]]
 [[`BOOST_STATIC_CONSTEXPR`][
 This is a shortcut for `static BOOST_CONSTEXPR_OR_CONST`. For example, when defining const expr variables replace:
 ``