Pre-import version statements

[Chris Piker]

Hi D

In my C code I used to typically put the line:
```
#define _POSIX_C_SOURCE 200112L
```
in the source before importing any standard library headers.  Is 
there something equivalent for phobos?  Say 
`version(phobos2.100)` or similar?

I don't particularly need this functionality, just checking to 
see if there as a formal way to denote what library standard the 
application code is expect to work against.

Thanks,