Stop cabal from complaining about LICENSE
Switch to cabal-dev

Of course, you can still build with regular cabal, it's just that the
Makefile will no longer help you with that.
clean harder

Using cabal-dev adds a lot of compiled files to the folder, but it takes
a long time to generate them, so you probably don't want to remove them
even after a "make clobber". If you really do want to remove them,
though, now there is "make distclean"!