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

[Adam Vogt]

Hi,

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

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.

Cheers,
Adam