Build script relies on third party to import Context before Fiber

[Kojoley]

Fiber build script uses <segmented-stacks> feature https://github.com/boostorg/fiber/blob/2cb72f5dcefdeffbb36636234e6ccb36282f8ae3/build/Jamfile.v2#L23-L26 which is defined in Context library build script. It works currently by pure luck that boost-cpp.jam (main Boost build script) was unintentionally consistently importing Context before Fiber.

Possible solutions:

1. If -fsplit-stack -DBOOST_USE_SEGMENTED_STACKS is not needed by Fiber itself -- we can just delete these lines and issue will be resolved.
2. If -fsplit-stack -DBOOST_USE_SEGMENTED_STACKS is needed by Fiber -- we can put there in Context usage requirements and Fiber will get them automatically by using <library>/boost/context//boost_context.