Externals: Update libusb to version 1.0.21

1.0.21 adds a usbdk backend, which may be helpful as it could be
used in the future to work around the missing isochronous transfer
support for WinUSB backends.

Here is the full changelog (from libusb).

2016-10-01: v1.0.21:
* Core: Refactor code related to transfer flags and timeout handling
* Darwin: Ignore root hub simulation devices
* Darwin: Improved support for OS X El Capitan
* Darwin: Work around devices with buggy endpoint descriptors
* Darwin: Do not use objc_registerThreadWithCollector after its deprecation
* Darwin: Use C11 atomics on 10.12+ as the OS atomics are now deprecated
* Linux: Support preallocating kernel memory for zerocopy USB
* Linux: Deal with receiving POLLERR before all transfers have completed
* Solaris: Add solaris backend
* Windows: Add Visual Studio 2015 support
* Windows: Add usbdk backend
* Prevent attempts to recursively handle events
* Fix race condition in handle_timeout()
* Allow transferred argument to be optional in bulk APIs
* Various other bug fixes and improvements

Externals/libusb/msvc/ddk_build.cmd

@@ -3,20 +3,29 @@
 ::# you can pass the following arguments (case insensitive):
 ::# - "DLL" to build a DLL instead of a static library
 ::# - "/MT" to build a static library compatible with MSVC's /MT option (LIBCMT vs MSVCRT)
+::# - "USBDK" to build with UsbDk backend
 
 if Test%BUILD_ALT_DIR%==Test goto usage
 
 ::# process commandline parameters
 set TARGET=LIBRARY
 set STATIC_LIBC=
+set WITH_USBDK=
 set version=1.0
 set PWD=%~dp0
 set BUILD_CMD=build -bcwgZ -M2
 
+:more_args
+
 if "%1" == "" goto no_more_args
 ::# /I for case insensitive
 if /I Test%1==TestDLL set TARGET=DYNLINK
 if /I Test%1==Test/MT set STATIC_LIBC=1
+if /I Test%1==TestUSBDK set WITH_USBDK=1
+
+shift
+goto more_args
+
 :no_more_args
 
 cd ..\libusb\os