Don't define timespec in VS 2015 builds.

VS 2015 includes time.h from stdlib.h which leads to redefinition
warnings when threads_windows.h defines the timespec struct.

Ideally threads_windows.h shouldn't be defining CRT structs at all, but
I am doing the minimum scoped change to move VS 2015 development
forwards.

_MSC_VER == 1900 represents the VC++ 2015 compiler.

R=reillyg@chromium.org
BUG=440500

Review URL: https://codereview.chromium.org/1067093002

Cr-Commit-Position: refs/heads/master@{#324133}

third_party/libusb/src/libusb/os/threads_windows.h

@@ -44,10 +44,13 @@ typedef struct usbi_cond_t_ usbi_cond_t;
 #if (!defined(HAVE_STRUCT_TIMESPEC) && !defined(_TIMESPEC_DEFINED))
 #define HAVE_STRUCT_TIMESPEC 1
 #define _TIMESPEC_DEFINED 1
+#if _MSC_VER < 1900
+// VS 2015 and above include <time.h> in stdlib.h
 struct timespec {
 		long tv_sec;
 		long tv_nsec;
 };
+#endif
 #endif /* HAVE_STRUCT_TIMESPEC | _TIMESPEC_DEFINED */
 
 // We *were* getting ETIMEDOUT from pthread.h: