Cygwin problem caused by config.hpp defining BOOST_USE_UCONTEXT

https://github.com/boostorg/context/blob/fb0f3eab4d14ce2de4cf3a5446c8a0f735a9282d/include/boost/context/detail/config.hpp#L33-L35

defines `BOOST_USE_UCONTEXT` on Cygwin. This causes Fiber to fail to build, because it tries to use ucontext, but Context is built with fcontext.

If Context is built with ucontext instead, Fiber builds, but then Coroutine fails, because it needs fcontext:

https://github.com/boostorg/coroutine/blob/4e47f98d44ad0d5d487788ed6eb4d347489e5483/src/detail/coroutine_context.cpp#L68

Should `config.hpp` be defining `BOOST_USE_UCONTEXT` on Cygwin?