Convert the rest of the codebase to C89.

Update build system to enforce -std=c89 -pedantic.

src/util.h

@@ -84,8 +84,21 @@ static SECP256K1_INLINE void *checked_malloc(size_t size) {
 # endif
 #endif
 
+#if defined(_WIN32)
+# define I64FORMAT "I64d"
+# define I64uFORMAT "I64u"
+#else
+# define I64FORMAT "lld"
+# define I64uFORMAT "llu"
+#endif
+
 #if defined(HAVE___INT128)
-typedef unsigned __int128 uint128_t;
+# if defined(__GNUC__)
+#  define SECP256K1_GNUC_EXT __extension__
+# else
+#  define SECP256K1_GNUC_EXT
+# endif
+SECP256K1_GNUC_EXT typedef unsigned __int128 uint128_t;
 #endif
 
 #endif