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/config.h

@@ -5,10 +5,18 @@
 #error "Please make sure the msvc/ directory is removed from your build path."
 #endif
 
+/* Visual Studio 2015 and later defines timespec */
+#if defined(_MSC_VER) && (_MSC_VER >= 1900)
+#define _TIMESPEC_DEFINED 1
+#endif
+
 /* Disable: warning C4200: nonstandard extension used : zero-sized array in struct/union */
 #pragma warning(disable:4200)
 /* Disable: warning C6258: Using TerminateThread does not allow proper thread clean up */
 #pragma warning(disable: 6258)
+/* Disable: warning C4996: 'GetVersionA': was declared deprecated */
+#pragma warning(disable: 4996)
+
 #if defined(_PREFAST_)
 /* Disable "Banned API" errors when using the MS's WDK OACR/Prefast */
 #pragma warning(disable:28719)