### Please describe the problem.
Building on Linux, with a particular combination of flags, failed due to missing `async`.

### What steps will reproduce the problem?
1. Configure with the following flag combination

        cryptohash -quvi -feed tdfa -testsuite -android production -dns -xmpp -pairing -webapp -assistant dbus inotify -webdav s3

2. Attempt to build and you'll get an error on line 16 of `Utility/Batch.hs` because `Control.Concurrent.Async` isn't available.

### What version of git-annex are you using? On what operating system?
Version 4.20131024 on Linux

### Please provide any additional information below.

This is the patch I applied to `git-annex.cabal`:

                 CPP-Options: -DWITH_KQUEUE
                 C-Sources: Utility/libkqueue.c
     
    +  if os(linux)
    +    Build-Depends: async
    +
       if os(linux) && flag(Dbus)
         Build-Depends: dbus (>= 0.10.3)
         CPP-Options: -DWITH_DBUS

> Feel async is core enough it should depend on it unconditionally.
> [[done]] --[[Joey]]
