A 'lazy' compilation mode to make writting multiplatform code easier without having to clutter the project

[Richard (Rikki) Andrew Cattermole]

Alternative approach:

```d
module rt.event;

version(Windows)
	public import ... : poll;
else version(...)
	public import ... : poll;
else
	static assert(0, "I don't know how to handle event loop here");
```

```d
module ...;
version(Windows):

void poll() {

}
```