Tests: make unit tests work on macOS Sierra

Fixes #5455.

test/unit/helpers.lua

@@ -36,6 +36,7 @@ local function filter_complex_blocks(body)
             or string.find(line, "_ISwupper", 1, true)
             or string.find(line, "msgpack_zone_push_finalizer")
             or string.find(line, "msgpack_unpacker_reserve_buffer")
+            or string.find(line, "UUID_NULL")  -- static const uuid_t UUID_NULL = {...}
             or string.find(line, "inline _Bool")) then
       result[#result + 1] = line
     end

test/unit/preprocess.lua

@@ -105,7 +105,12 @@ local Gcc = {
    '-DINCLUDE_GENERATED_DECLARATIONS',
 
    -- Needed for FreeBSD
-   '-D "_Thread_local="'
+   '-D "_Thread_local="',
+
+   -- Needed for macOS Sierra
+   '-D "_Nullable="',
+   '-D "_Nonnull="',
+   '-U__BLOCKS__',
   }
 }