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/libusb_sources

@@ -12,8 +12,12 @@ USE_LIBCMT=1
 USE_MSVCRT=1
 !ENDIF
 
+!IFDEF WITH_USBDK
+BACKEND_DEFINES="/DUSE_USBDK"
+!ENDIF
+
 INCLUDES=..;..\..\msvc;$(DDK_INC_PATH)
-C_DEFINES= $(C_DEFINES) $(LIBUSB_DEFINES) /DDDKBUILD
+C_DEFINES= $(C_DEFINES) $(LIBUSB_DEFINES) $(BACKEND_DEFINES) /DDDKBUILD
 
 # http://jpassing.com/2009/10/21/ltcg-issues-with-the-win7amd64-environment-of-wdk-7600/
 # prevents the following error when using the 64 bit static lib with Visual Studio 2010:
@@ -34,5 +38,7 @@ SOURCES=..\core.c \
 	..\hotplug.c \
 	threads_windows.c \
 	poll_windows.c \
-	windows_usb.c \
+	windows_winusb.c \
+	windows_usbdk.c \
+	windows_nt_common.c \
 	..\libusb-1.0.rc