Remove unneeded stuff from nt/inc/sys/time.h

* nt/inc/sys/time.h (_TIMEVAL_DEFINED, struct timevat, timerisset)
(timercmp, timerclear): Don't define.  Instead, include the system
header sys/time.h, and add only the interval timers stuff.  This
avoids compiler warnings about 'gettimeofday's prototype, and also
avoids redefinition of macros from system headers.

nt/inc/sys/time.h

 #ifndef SYS_TIME_H_INCLUDED
 #define SYS_TIME_H_INCLUDED
 
-/*
- * sys/time.h either doesn't exist on Windows, or doesn't necessarily
- * have the below stuff.
- */
-
-/* The guards are for MinGW64, which defines these structs on its
-   system headers which are included by ms-w32.h.  */
-/* Allow inclusion of sys/time.h and winsock2.h in any order.  Needed
-   for running the configure test, which is only relevant to MinGW.  */
-#ifndef _TIMEVAL_DEFINED
-#define _TIMEVAL_DEFINED
-struct timeval
-{
-  long		tv_sec;		/* seconds */
-  long		tv_usec;	/* microseconds */
-};
-#define timerisset(tvp)  ((tvp)->tv_sec || (tvp)->tv_usec)
-#define timercmp(tvp, uvp, cmp) \
-        (((tvp)->tv_sec != (uvp)->tv_sec) ? \
-        ((tvp)->tv_sec cmp (uvp)->tv_sec) : \
-        ((tvp)->tv_usec cmp (uvp)->tv_usec))
-#define timerclear(tvp)  (tvp)->tv_sec = (tvp)->tv_usec = 0
-#endif /* _TIMEVAL_DEFINED */
+#include_next <sys/time.h>
 
 #define ITIMER_REAL      0
 #define ITIMER_PROF      1