And fix so that configure script can find libusb headers. This makes
it possible to access smartcards with gpg/scdaemon when running as
root. It is not without flaws though, the gpg-agent daemon has to be
manually stopped sometimes to make the card appear. Also seems quite
hard to get some things to work when running gpg-agent and scdaemon as
root: I haven't been able to do remote forwarding or sig git commits.
