Externals: Update libusb to 1.0.23-rc1

Now has support for isochronous transfers in the WinUSB backend,
which may or may not work better than the UsbDk backend.

Externals/libusb/README.md

@@ -0,0 +1,32 @@
+# libusb
+
+[![Build Status](https://travis-ci.org/libusb/libusb.svg?branch=master)](https://travis-ci.org/libusb/libusb)
+[![Build status](https://ci.appveyor.com/api/projects/status/xvrfam94jii4a6lw?svg=true)](https://ci.appveyor.com/project/LudovicRousseau/libusb)
+[![Coverity Scan Build Status](https://scan.coverity.com/projects/2180/badge.svg)](https://scan.coverity.com/projects/libusb-libusb)
+
+libusb is a library for USB device access from Linux, macOS,
+Windows, OpenBSD/NetBSD and Haiku userspace.
+It is written in C (Haiku backend in C++) and licensed under the GNU
+Lesser General Public License version 2.1 or, at your option, any later
+version (see [COPYING](COPYING)).
+
+libusb is abstracted internally in such a way that it can hopefully
+be ported to other operating systems. Please see the [PORTING](PORTING)
+file for more information.
+
+libusb homepage:
+http://libusb.info/
+
+Developers will wish to consult the API documentation:
+http://api.libusb.info
+
+Use the mailing list for questions, comments, etc:
+http://mailing-list.libusb.info
+
+- Hans de Goede <hdegoede@redhat.com>
+- Xiaofan Chen <xiaofanc@gmail.com>
+- Ludovic Rousseau <ludovic.rousseau@gmail.com>
+- Nathan Hjelm <hjelmn@cs.unm.edu>
+- Chris Dickens <christopher.a.dickens@gmail.com>
+
+(Please use the mailing list rather than mailing developers directly)