Fix #1110, 32-bit build failure on Mac OSX (#1112)

Using explicit 64-bit type in conditional in platforms above 32-bits
This appears to be necessary on Mac OSX as std::conditional does not appear to short circuit and evaluates the third template arg
Making the third template arg be 64 bits explicitly works around this problem and will work on both 32 bit and 64+ bit platforms.

db/column_family.cc

@@ -149,7 +149,7 @@ ColumnFamilyOptions SanitizeOptions(const DBOptions& db_options,
   result.comparator = icmp;
   size_t clamp_max = std::conditional<
       sizeof(size_t) == 4, std::integral_constant<size_t, 0xffffffff>,
-      std::integral_constant<size_t, 64ull << 30>>::type::value;
+      std::integral_constant<uint64_t, 64ull << 30>>::type::value;
   ClipToRange(&result.write_buffer_size, ((size_t)64) << 10, clamp_max);
   // if user sets arena_block_size, we trust user to use this value. Otherwise,
   // calculate a proper value from writer_buffer_size;