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* Add ability to force use of the system or builtin libusb-1.0 library.
    Use: gem install libusb -- --enable-system-libusb / --disable-system-libusb
* Update to libusb-1.0.20.
* Build Windows binary gems per rake-compiler-dock.
* Fix memory leak in Context#pollfds and use libusb_free_pollfds() if available.