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

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