Clean up int128_t macro gating

Summary: There is inconsistent use of `#ifdef` and `#if`. Cleaning this up

Reviewed By: yfeldblum

Differential Revision: D3523561

fbshipit-source-id: 23f62d632efeb1b9b137b3e3582c37aae591d901

folly/Conv.cpp

@@ -69,7 +69,7 @@ static_assert(sizeof(unsigned long long) >= 8,
               "Wrong value for MaxString<unsigned long long>::value"
               ", please update.");
 
-#ifdef FOLLY_HAVE_INT128_T
+#if FOLLY_HAVE_INT128_T
 template <> const char *const MaxString<__uint128_t>::value =
   "340282366920938463463374607431768211455";
 #endif

folly/Conv.h

@@ -702,7 +702,7 @@ struct HasLengthEstimator : std::false_type {};
 template <class Src>
 constexpr typename std::enable_if<
   !std::is_fundamental<Src>::value
-#ifdef FOLLY_HAVE_INT128_T
+#if FOLLY_HAVE_INT128_T
   // On OSX 10.10, is_fundamental<__int128> is false :-O
   && !std::is_same<__int128, Src>::value
   && !std::is_same<unsigned __int128, Src>::value