Version declaration proposal

[Don]

Daniel Keep wrote:
> I actually quite like the idea of being able to specify which versions
> you're allowed to set from the command-line.  The only thing I can think
> of which this doesn't cover is how this affects (if indeed it does)
> integer versions.
> 
>   -- Daniel

I reckon those integer versions are a misfeature to drop.
Compile this with version=3.
---
version(2) {
   pragma(msg, "surprise!");
} else version(3) {
   pragma(msg, "I bet you were expecting this.");
}