Boost.Context context factory
* This is a portable replacement for ucontext.h (makecontext,
swapcontext, getcontext, setcontext) which is not in recent POSIX
specs and was apparently removed from Mac OS X since 10.9. POSIX
2008 still has ucontext_t however (moved into signal.h).
* Currently works only for Boost <= 1.55 (the Boost.Context API
changed slightly afterwards).
* Does not work with MC because we currently depend on the contexts
being ucontext_t (we'd have to convert fcontext into ucontext_t).
* No support for parallel execution support (yet).