Ensure that WINVER is defined early enough, so other system headers
won't fall back to their default value.  Specifically, this solves a
build failure (-Werror) with mingw-w64 ("WINVER redefined").

In a C file, that is too late, because _mingw.h defaults to 0x0502,
and ours would be illegal redefintion.

I've added PIPE_EVENT_SHUTDOWN because calling g_main_loop_quit() do not work when called from another thread.
Main thread was sleeping in g_poll() so I needed some way to wake it up.

By some strange reason call close(event_pipe[0]) in event_pipe_deinit() hangs.
In current implementation that code never reached so that was not a problem :-)
I've added a conditional to leave event_pipe[0] open on Win32.