* configure.ac.in: vlc can now be built under cygwin with or without the
   unix emulation layer (without if you use CC="gcc -mno-cygwin").
* INSTALL.win32: doc update for cygwin build.
* modules/video_output/directx/events.c, modules/video_output/directx/directx.c,
   modules/audio_output/directx.c: compilation fixes for cygwin.