Cygwin build is broken

[corngood]

I have a series of PRs required to fix cygwin builds of boost. There's one for each of the submodule changes here:

master...corngood:boost:cygwin

These are based on the out-of-date downstream patches in cygwin:

https://cygwin.com/cgit/cygwin-packages/boost/tree/

This issue is to track them in one place:

• Fix cygwin build. asio#463
• Don't default to ucontext on cygwin. context#322
• Fix cygwin build. process#533
• Disable from_exception on cygwin. stacktrace#219
• Use BOOST_POSIX_API on cygwin. system#137