This disables hotplug support for Linux. Since hotplug support is already in the master branch, I would like to enable it. So I would rather set this flag only conditionally. We could do the check for libudev in extconf.rb by have_library/have_header similar to [libusbx-configure.ac](https://github.com/libusbx/libusbx/blob/master/configure.ac#L96). Any objections?