configure: add endian check

Will remove guesswork and manual hacking in the OS headers.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
diff --git a/os/os.h b/os/os.h
index a14d7f0..fb544a1 100644
--- a/os/os.h
+++ b/os/os.h
@@ -165,7 +165,7 @@
 #endif
 
 #ifndef FIO_HAVE_BYTEORDER_FUNCS
-#ifdef FIO_LITTLE_ENDIAN
+#ifdef CONFIG_LITTLE_ENDIAN
 #define __le16_to_cpu(x)		(x)
 #define __le32_to_cpu(x)		(x)
 #define __le64_to_cpu(x)		(x)