Fix native extension build on mswin

[hsbt]

Installing io-event on mswin (x64-mswin64_140) fails because ext/io/event/interrupt.c includes unistd.h, which MSVC does not ship. Guarding that include is not enough. The existing rb_w32_pipe fallback is skipped because ruby's mswin config.h defines HAVE_PIPE, while the pipe macro in ruby/win32.h is only visible when building ruby itself. extconf.rb also never checked for ruby/win32.h, so that fallback was dead code.

I made extconf.rb detect unistd.h and ruby/win32.h, and changed the fallback to define pipe whenever the macro is missing. The extension now builds with MSVC 14.51, and IO::Event::Selector::Select works for a basic io_wait on a pipe. This is independent of the IOCP selector in #188, which leaves interrupt.c untouched.

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