BOOST_UNLIKELY and boost::system::error_code

Could you please modify the `BOOST_LIKELY` and `BOOST_UNLIKELY` macros in the following way:

```cpp
#define BOOST_LIKELY(x) (__builtin_expect(!!(x), 1))
#define BOOST_UNLIKELY(x) (__builtin_expect(!!(x), 0))

```

1. I cannot easily use `BOOST_UNLIKELY` with `boost::system::error_code`. Compiler complains

```
error: cannot convert ‘boost::system::error_code’ to ‘long int’ for argument ‘1’ to ‘long int __builtin_expect(long int, long int)’
[build] if (BOOST_UNLIKELY (ec)) {
[build] ^
```

To workaround the problem I should use macro like following:
```
if (BOOST_UNLIKELY (!!ec))
```
... which looks weird and causes unwanted questions during the code review.

2. I have to add extra braces:

```cpp
if (BOOST_UNLIKELY (!!ec))
^ -- here ^ --- and here
```
... instead of:

```cpp
if BOOST_UNLIKELY (ec)
{
}
```