Typedef (u)int128_t only when they're not provided by the compiler

src/util.h

@@ -260,14 +260,20 @@ static SECP256K1_INLINE void secp256k1_int_cmov(int *r, const int *a, int flag)
 # define SECP256K1_WIDEMUL_INT128 1
 #elif defined(USE_FORCE_WIDEMUL_INT64)
 # define SECP256K1_WIDEMUL_INT64 1
-#elif defined(__SIZEOF_INT128__)
+#elif defined(UINT128_MAX) || defined(__SIZEOF_INT128__)
 # define SECP256K1_WIDEMUL_INT128 1
 #else
 # define SECP256K1_WIDEMUL_INT64 1
 #endif
 #if defined(SECP256K1_WIDEMUL_INT128)
+# if !defined(UINT128_MAX) && defined(__SIZEOF_INT128__)
 SECP256K1_GNUC_EXT typedef unsigned __int128 uint128_t;
 SECP256K1_GNUC_EXT typedef __int128 int128_t;
+#define UINT128_MAX ((uint128_t)(-1))
+#define INT128_MAX ((int128_t)(UINT128_MAX >> 1))
+#define INT128_MIN (-INT128_MAX - 1)
+/* No (U)INT128_C macros because compilers providing __int128 do not support 128-bit literals.  */
+# endif
 #endif
 
 #endif /* SECP256K1_UTIL_H */