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/Xcode/debug.xcconfig

@@ -0,0 +1,29 @@
+//
+// libusb Xcode configuration file
+// Copyright © 2012 Pete Batard <pete@akeo.ie>
+// For more information, please visit: <http://libusb.info>
+//
+// This library is free software; you can redistribute it and/or
+// modify it under the terms of the GNU Lesser General Public
+// License as published by the Free Software Foundation; either
+// version 2.1 of the License, or (at your option) any later version.
+//
+// This library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+// Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public
+// License along with this library; if not, write to the Free Software
+// Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+
+#include "common.xcconfig"
+
+// Embed debug symbols in binary itself.
+DEBUG_INFORMATION_FORMAT = dwarf
+
+// No optimizations in debug.
+GCC_OPTIMIZATION_LEVEL = 0
+
+// 
+GCC_PREPROCESSOR_DEFINITIONS = $(inherited) DEBUG=1