Build assertion against conditions which cannot be evaluated.

According to the documentation:

> Your compile will fail if the condition isn't true, or can't be evaluated by the compiler.  

I  find no tests for the latter case, so write one myself:

```C
    int val = 0;
    BUILD_ASSERT(val != 0);
    BUILD_ASSERT(val == 0);
```
It compiles without any complaining.
