Specify OS version

Is there some way to extend my cabal file and specify which _version_ of os is currently in use and load specific modules?

AFAIK, Cabal's `conditional` doesn't provide such mechanism, but something like

``` haskell
if os(osx) && ver(=10.10)
hs-source-dirs: osx10
if os(osx) && ver(<=10.9)
hs-source-dirs: osx9
```

will be nice, where `ver` is keyword for version of os

If there is no such thing, will it be acceptable if I submit patch for it?

The issue does not name any files, tests, or entry points. Start by locating Cabal's existing conditional configuration handling and OS detection, then determine how version comparisons and module selection should be specified and tested. Done means a documented, accepted mechanism for selecting settings by OS version.

Written by the indexing model from the issue text.