Add fallback LE/BE for architectures with known endianness

src/util.h

@@ -186,6 +186,26 @@ static SECP256K1_INLINE void *manual_alloc(void** prealloc_ptr, size_t alloc_siz
 #if defined(_MSC_VER) && defined(_WIN32) && !defined(SECP256K1_LITTLE_ENDIAN)
 # define SECP256K1_LITTLE_ENDIAN
 #endif
+#if !defined(SECP256K1_LITTLE_ENDIAN) && !defined(SECP256K1_BIG_ENDIAN)
+/* Try to derive from arch/compiler-specific macros. Code taken from
+   https://github.com/rofl0r/endianness.h/blob/9853923246b065a3b52d2c43835f3819a62c7199/endianness.h#L52L73
+*/
+# if defined(_X86_) || defined(__x86_64__) || defined(__i386__) || \
+     defined(__i486__) || defined(__i586__) || defined(__i686__) || \
+     defined(__MIPSEL) || defined(_MIPSEL) || defined(MIPSEL) || \
+     defined(__ARMEL__) || \
+     (defined(__LITTLE_ENDIAN__) && __LITTLE_ENDIAN__ == 1) || \
+     (defined(_LITTLE_ENDIAN) && _LITTLE_ENDIAN == 1) || \
+     defined(_M_IX86) || defined(_M_AMD64) /* MSVC */
+#  define SECP256K1_LITTLE_ENDIAN
+# endif
+# if defined(__MIPSEB) || defined(_MIPSEB) || defined(MIPSEB) || \
+     defined(__MICROBLAZEEB__) || defined(__ARMEB__) || \
+     (defined(__BIG_ENDIAN__) && __BIG_ENDIAN__ == 1) || \
+     (defined(_BIG_ENDIAN) && _BIG_ENDIAN == 1)
+#  define SECP256K1_BIG_ENDIAN
+# endif
+#endif
 #if defined(SECP256K1_LITTLE_ENDIAN) == defined(SECP256K1_BIG_ENDIAN)
 # error Please make sure that either SECP256K1_LITTLE_ENDIAN or SECP256K1_BIG_ENDIAN is set, see src/util.h.
 #endif