Use additional system macros to figure out endianness

Also permit it being overridden by explicitly passing SECP256K1_{BIG,LITTLE}_ENDIAN

src/util.h

@@ -176,16 +176,27 @@ static SECP256K1_INLINE void *manual_alloc(void** prealloc_ptr, size_t alloc_siz
 # define SECP256K1_GNUC_EXT
 #endif
 
-#if defined(__BYTE_ORDER__)
-# if defined(__ORDER_LITTLE_ENDIAN__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__ && !defined(SECP256K1_LITTLE_ENDIAN)
+/* If SECP256K1_{LITTLE,BIG}_ENDIAN is not explicitly provided, infer from various other system macros. */
+#if !defined(SECP256K1_LITTLE_ENDIAN) && !defined(SECP256K1_BIG_ENDIAN)
+/* Inspired by https://github.com/rofl0r/endianness.h/blob/9853923246b065a3b52d2c43835f3819a62c7199/endianness.h#L52L73 */
+# if (defined(__BYTE_ORDER__) && defined(__ORDER_LITTLE_ENDIAN__) && __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__) || \
+     defined(_X86_) || defined(__x86_64__) || defined(__i386__) || \
+     defined(__i486__) || defined(__i586__) || defined(__i686__) || \
+     defined(__MIPSEL) || defined(_MIPSEL) || defined(MIPSEL) || \
+     defined(__ARMEL__) || defined(__AARCH64EL__) || \
+     (defined(__LITTLE_ENDIAN__) && __LITTLE_ENDIAN__ == 1) || \
+     (defined(_LITTLE_ENDIAN) && _LITTLE_ENDIAN == 1) || \
+     defined(_M_IX86) || defined(_M_AMD64) || defined(_M_ARM) /* MSVC */
 #  define SECP256K1_LITTLE_ENDIAN
-# elif defined(__ORDER_BIG_ENDIAN__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__ && !defined(SECP256K1_BIG_ENDIAN)
+# endif
+# if (defined(__BYTE_ORDER__) && defined(__ORDER_BIG_ENDIAN__) && __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__) || \
+     defined(__MIPSEB) || defined(_MIPSEB) || defined(MIPSEB) || \
+     defined(__MICROBLAZEEB__) || defined(__ARMEB__) || defined(__AARCH64EB__) || \
+     (defined(__BIG_ENDIAN__) && __BIG_ENDIAN__ == 1) || \
+     (defined(_BIG_ENDIAN) && _BIG_ENDIAN == 1)
 #  define SECP256K1_BIG_ENDIAN
 # endif
 #endif
-#if defined(_MSC_VER) && defined(_WIN32) && !defined(SECP256K1_LITTLE_ENDIAN)
-# define SECP256K1_LITTLE_ENDIAN
-#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