update hl_sys_time to support OS below Windows 8 (#285)

src/std/sys.c

@@ -164,17 +164,26 @@ HL_PRIM void hl_sys_exit( int code ) {
 HL_PRIM double hl_sys_time() {
 #ifdef HL_WIN
 	#define EPOCH_DIFF	(134774*24*60*60.0)
+	static double time_diff = 0.;
 	static double freq = 0.;
 	LARGE_INTEGER time;
+	LARGE_INTEGER start_time;
 	FILETIME ft;
+
 	if( freq == 0 ) {
 		QueryPerformanceFrequency(&time);
 		freq = (double)time.QuadPart;
 	}
-	GetSystemTimePreciseAsFileTime(&ft);
-	time.LowPart = ft.dwLowDateTime;
-	time.HighPart = ft.dwHighDateTime;
-	return ((double)time.QuadPart) / freq - EPOCH_DIFF;
+	QueryPerformanceCounter(&time);
+
+	if( time_diff == 0 ) {
+		GetSystemTimeAsFileTime(&ft);
+		start_time.LowPart = ft.dwLowDateTime;
+		start_time.HighPart = ft.dwHighDateTime;
+		time_diff = ((double)start_time.QuadPart - (double)time.QuadPart) / freq - EPOCH_DIFF;
+	}
+
+	return time_diff + ((double)time.QuadPart) / freq;
 #else
 	struct timeval tv;
 	if( gettimeofday(&tv,NULL) != 0 )