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

[Don Stewart]

vogt.adam:
> Hi,
> 
> I want to add '-fwarn-tabs' to the warnings run when testing patches?  (and 
> clean up the couple uses of tabs).

Seems reasonable.

> The warning is documented in ghc-6.8, missing in ghc-6.6 (but it could still be
> there?), ghc-6.6 does not support the current darcs contrib, due to various 
> pragmas in use, such as -XDeriveDataTypeable, among others.
> 
> Changing the -fglasgow-exts flags to more specific pragmas should carry the 
> same risks too.

--- Don