configure: Check for sys/time.h

Apparently this include is needed on some systems for building the
poll fallback (for the timeval struct for select?), but it isn't
available on all systems. Thus only include it if it exists.

Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/os_support.c

@@ -58,7 +58,9 @@ int ff_win32_open(const char *filename_utf8, int oflag, int pmode)
 #if CONFIG_NETWORK
 #include <fcntl.h>
 #if !HAVE_POLL_H
+#if HAVE_SYS_TIME_H
 #include <sys/time.h>
+#endif
 #if HAVE_WINSOCK2_H
 #include <winsock2.h>
 #elif HAVE_SYS_SELECT_H