lavu: add av_usleep() function

This function implements a delay using the first available
of the following functions:

- nanosleep()
- usleep()
- Sleep() (Windows)

The conditional #includes in time.c are simplified by including
unistd.h and windows.h whenever they are available rather than
having these lines triggered by specific functions.

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavutil/time.c

@@ -20,13 +20,19 @@
 
 #include <stddef.h>
 #include <stdint.h>
+#include <time.h>
 #if HAVE_GETTIMEOFDAY
 #include <sys/time.h>
-#elif HAVE_GETSYSTEMTIMEASFILETIME
+#endif
+#if HAVE_UNISTD_H
+#include <unistd.h>
+#endif
+#if HAVE_WINDOWS_H
 #include <windows.h>
 #endif
 
 #include "libavutil/time.h"
+#include "error.h"
 
 int64_t av_gettime(void)
 {
@@ -44,3 +50,19 @@ int64_t av_gettime(void)
     return -1;
 #endif
 }
+
+int av_usleep(unsigned usec)
+{
+#if HAVE_NANOSLEEP
+    struct timespec ts = { usec / 1000000, usec % 1000000 * 1000 };
+    while (nanosleep(&ts, &ts) < 0 && errno == EINTR);
+    return 0;
+#elif HAVE_USLEEP
+    return usleep(usec);
+#elif HAVE_SLEEP
+    Sleep(usec / 1000);
+    return 0;
+#else
+    return AVERROR(ENOSYS);
+#endif
+}