timersub macro needed for timing in iofunc.c for win32

macros/gr_pwin32.m4

@@ -104,6 +104,18 @@ struct timezone {
 int gettimeofday(struct timeval *tv, struct timezone *tz);
 #endif
 
+#ifndef timersub
+# define timersub(a, b, result) \
+    do { \
+        (result)->tv_sec = (a)->tv_sec - (b)->tv_sec; \
+        (result)->tv_usec = (a)->tv_usec - (b)->tv_usec; \
+        if ((result)->tv_usec < 0) { \
+            --(result)->tv_sec; \
+            (result)->tv_usec += 1000000; \
+        } \
+    } while (0)
+#endif
+
 #ifndef HAVE_SSIZE_T
 typedef size_t ssize_t;
 #endif