Add configure script

Some older installs don't have __kernel_rwf_t in linux/fs.h, so
add a check for that.

Signed-off-by: Jens Axboe <axboe@kernel.dk>

src/io_uring.c

@@ -5,6 +5,7 @@
 #include <errno.h>
 #include <string.h>
 
+#include "compat.h"
 #include "io_uring.h"
 #include "liburing.h"
 #include "barrier.h"