w32: Make scdaemon build again due to libusb problem.

* configure.ac: Add hack to disable libusb for Windows.  Also use
$host instead of $target in the switch

The new test for libusb does not support cross-compiling. As a quick
workaround we disable libusb for Windows because we can't use it anyway.

• Signed-off-by: Werner Koch <wk@gnupg.org>