Change default platform to something that don't change when it isn't nee...

...ded.

News.txt
- Better default compiledir name

@lamblin and @abergeron can both of you review comment on this before? Currently the compiledir change too often. I think that this version would work on windows and mac. I'll test on windows. @abergeron can you test on mac?
This looks very distribution-specific.
For instance, on ubuntu 14.10:
```
>>> platform.release()
'3.13.0-39-generic'
```
So maybe we can accept `len(sp) > 2`, since we only care about `sp[0]`?
Or only do that for Linux?