Use double-bracket escaping

Use double-bracket escaping per Akim's suggestion as it makes the code
snippet used easier to understand directly (for a C++ programmer).

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Boost.Coroutine: no direct link-dependency on Boost.Context

Starting from Boost 1.54 it is no longer required to link directly to
Boost.Context as the usage of Context is moved out of Coroutine's
headers and into its shared library.

Signed-off-by: Giel van Schijndel <me@mortis.eu>
Boost.Coroutine: update for Boost 1.55

In Boost 1.55 a dependency on Boost.System is added to Boost.Coroutine
(error_code class). Ensure we link with that.

Signed-off-by: Giel van Schijndel <me@mortis.eu>