Improve Haskell configure options and detection

This patch cleans up  the Haskell library detection and defaults.

First, it makes the base compiler/libraries required, per the email discussion.

It then adds two new small autoconf macros, on to check for a required
Haskell library and one to do custom action based on test results. We
use these macros to cleanup and simplify a bit the module detection:

- rapi, confd, and split query are auto detected and enabled if _all_
  required libraries are present
- unittests are enabled if _all_ required libraries are present

The patch also updates the documentation regarding required libraries.

After this patch, base Ganeti fully buildable on Debian Squeeze/Ubuntu
Lucid.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>