[xmonad] Enabling -fwarn-tabs for commit test (for contrib)

[Adam Vogt]

* On Tuesday, June 23 2009, Spencer Janssen wrote:

>I think we were waiting on -fwarn-tabs until we stopped supporting 6.6.  
>Now
>that we've done that we should absolutely activate -fwarn-tabs.
>
>In the case of -fglasgow-exts, do be careful.  I think there were some bugs
>between 6.8 and 6.10 and extension flags.

I've updated most uses of -fglasgow-exts, except in the XMonad.Config 
namespace, and XMonad.Hooks.ManageDocks (since it could still be moved into 
core, where 6.6 compatiblity is desirable). vav tested it on ghc-6.8 too.

Thanks,
Adam