file: Only include unistd.h if it exists

It is included for the open/read/write/close functions. On
MSVC, where this header does not exist, the same functions
are provided by io.h, which is already included.

On windows, these functions are provided by io.h. Make sure
io.h is included if it exists, regardless of the setmode
function.
Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -1093,6 +1093,7 @@ HAVE_LIST="
     ibm_asm
     inet_aton
     inline_asm
+    io_h
     isatty
     isinf
     isnan
@@ -2879,6 +2880,7 @@ check_func_headers windows.h VirtualAlloc
 check_header dlfcn.h
 check_header dxva.h
 check_header dxva2api.h
+check_header io.h
 check_header malloc.h
 check_header poll.h
 check_header sys/mman.h

libavformat/file.c

@@ -22,10 +22,12 @@
 #include "libavutil/avstring.h"
 #include "avformat.h"
 #include <fcntl.h>
-#if HAVE_SETMODE
+#if HAVE_IO_H
 #include <io.h>
 #endif
+#if HAVE_UNISTD_H
 #include <unistd.h>
+#endif
 #include <sys/stat.h>
 #include <stdlib.h>
 #include "os_support.h"