How would you solve this problem: do an optional function call
depending on some version(X). If version(X) is not defined, there
should be no call and no extra code at -O0.
```
{
...
foo(); // either compiles to a function call, or to _nothing_.
...
}
```
In C, you could do something like:
```
#if X
void foo() {......}
#else
#define foo()
#endif
```
How would you do this in D?
I can think of `mixin(foo())` but there is probably a nicer way
that preserves normal function calling syntax.
Cheers,
Johan
