fix endianness detection for some versions of gcc

This fixes #254

src/headers/tomcrypt_cfg.h

@@ -168,15 +168,17 @@ LTC_EXPORT int   LTC_CALL XSTRCMP(const char *s1, const char *s2);
 
 /* endianness fallback */
 #if !defined(ENDIAN_BIG) && !defined(ENDIAN_LITTLE)
-  #if defined(__BYTE_ORDER) && __BYTE_ORDER == __BIG_ENDIAN || \
+  #if defined(_BYTE_ORDER) && _BYTE_ORDER == _BIG_ENDIAN || \
+      defined(__BYTE_ORDER) && __BYTE_ORDER == __BIG_ENDIAN || \
       defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ || \
-      defined(__BIG_ENDIAN__) || defined(_BIG_ENDIAN) || \
+      defined(__BIG_ENDIAN__) || \
       defined(__ARMEB__) || defined(__THUMBEB__) || defined(__AARCH64EB__) || \
       defined(_MIPSEB) || defined(__MIPSEB) || defined(__MIPSEB__)
     #define ENDIAN_BIG
-  #elif defined(__BYTE_ORDER) && __BYTE_ORDER == __LITTLE_ENDIAN || \
+  #elif defined(_BYTE_ORDER) && _BYTE_ORDER == _LITTLE_ENDIAN || \
+      defined(__BYTE_ORDER) && __BYTE_ORDER == __LITTLE_ENDIAN || \
       defined(__BYTE_ORDER__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ || \
-      defined(__LITTLE_ENDIAN__) || defined(_LITTLE_ENDIAN) || \
+      defined(__LITTLE_ENDIAN__) || \
       defined(__ARMEL__) || defined(__THUMBEL__) || defined(__AARCH64EL__) || \
       defined(_MIPSEL) || defined(__MIPSEL) || defined(__MIPSEL__)
     #define ENDIAN_LITTLE