More portable endianness check
https://www.austingroupbugs.net/view.php?id=162#c665

Index: sha1.c
--- sha1.c.orig
+++ sha1.c
@@ -29,7 +29,7 @@
 #include <string.h>
 #include <endian.h>
 
-#if __BYTE_ORDER == __BIG_ENDIAN
+#if BYTE_ORDER == BIG_ENDIAN
 # define SWAP(n) (n)
 #else
 # define SWAP(n) \