message: builtin_expect clang6 fix
diff: #define MLDB_PURE_FN __attribute__((__pure__))
  #define MLDB_CONST_FN __attribute__((__const__))
  #define MLDB_WEAK_FN __attribute__((__weak__))
  -#define MLDB_LIKELY(x) __builtin_expect((x), true)
  +#define MLDB_LIKELY(x) __builtin_expect(bool(x), true)
  #define MLDB_UNLIKELY(x) __builtin_expect((x), false)
  #define MLDB_DEPRECATED __attribute__((__deprecated__))
  #define MLDB_ALIGNED(x) __attribute__((__aligned__(x)))
  